The Immaculate Conception of the Blessed Virgin Mary is  harmonious with Holy Scripture, and is supported by both analogy and plausibility.

Luke 1:28 (RSV) And he came to her and said, “Hail, O favored one, the Lord is with you!” [in the RSV – Catholic edition, “favored one” is rendered as “full of grace”]

Many translations use “favor” here, yet even the great Baptist Greek scholar A. T. Robertson (Word Pictures in the New Testament, Vol. II, 13) agrees that the word involved (kecharitomene) means “full of grace which thou hast received”. It's derived from the Greek root, charis (literally, “grace”).

Kecharitomene has to do with God’s grace, as it is derived from the Greek root, charis (literally, “grace”). Thus, in the KJV, charis is translated “grace” 129 out of the 150 times that it appears. Presbyterian Greek scholar Marvin Vincent (Word Studies in the New Testament) noted that even Wycliffe and Tyndale (no enthusiastic supporters of the Catholic Church) both rendered kecharitomene in Luke 1:28 as “full of grace” and that the literal meaning was “endued with grace” (I, 259).

Likewise, well-known Protestant linguist W. E. Vine (Expository Dictionary of New Testament Words), defines it as “to endue with Divine favour or grace” (II, 171). All these men (except Wycliffe, who probably would have been, had he lived in the 16th century or after it) are Protestants, and so cannot be accused of Catholic translation bias.

Now, one might wonder, “what does it mean to be full of grace?” For St. Paul, grace is the antithesis and overcomer of sin:

Romans 6:14, 22 For sin will have no dominion over you, since you are not under law but under grace. . . . [22] But now that you have been set free from sin . . . the return you get is sanctification and its end, eternal life.

Moreover, we are saved by grace:

Ephesians 2:8 For by grace you have been saved through faith; and this is not your own doing, it is the gift of God

Acts 15:11 . . . we shall be saved through the grace of the Lord Jesus . . .

Romans 3:24 they are justified by his grace as a gift, through the redemption which is in Christ Jesus, 

Titus 2:11 For the grace of God has appeared for the salvation of all men,

Titus 3:7 so that we might be justified by his grace and become heirs in hope of eternal life. 

Therefore, it follows, I submit, that for a person to be full of grace is to both be saved and to be exceptionally, completely holy. Therefore, Mary is holy and sinless. The essence of the Immaculate Conception is sinlessness, and this is deduced from many biblical passages about the antithetical relation of grace to sin, and salvation and its accompanying sanctification to sin.

The only remaining question is: when did God apply this grace to Mary? We know she possessed it as a young woman, at the Annunciation. Catholics believe that God gave her the grace at her conception so as to avoid the original sin that she inevitably would have inherited, being human, but for God's preventive grace, which saved her from falling into the pit of sin. It was grace from God that couldn’t possibly have had anything to do with Mary’s personal merit.

But do we ever observe in the Bible, other persons being extraordinarily sanctified, even before their birth? Yes; for example, the prophets Isaiah and Jeremiah:

Isaiah 49:1, 5 . . . The LORD called me from the womb, . . . [5] And now the LORD says, who formed me from the womb to be his servant,  . . . (cf. Job 31:15, 18; Jud 16:17)

Jeremiah 1:5 Before I formed you in the womb I knew you, and before you were born I consecrated you; I appointed you a prophet to the nations. (cf. Sirach 49:7)

“Consecrated” or “sanctified” (KJV) in Jeremiah 1:5 is the Hebrew word quadash. According to Gesenius' Hebrew-Chaldee Lexicon of the Old Testament (p. 725), in this instance it meant “to declare any one holy.”

Jeremiah was thus consecrated or sanctified from the womb; possibly from conception. This is fairly analogous to the doctrine of the Immaculate Conception. We know Jeremiah was a very holy man, and perhaps even sinless. We also have New Testament evidence of such sanctification before birth (John the Baptist and St. Paul):

Luke 1:15 for he will be great before the Lord, . . . and he will be filled with the Holy Spirit, even from his mother's womb. (cf. 1:41, 44)

Galatians 1:15 . . . he who had set me apart before I was born, and had called me through his grace,

We know that John the Baptist was also a very holy man. Was he sinless? We can't know that for sure from the biblical data. St. Catherine of Siena, for one, believed that he never sinned (A Treatise of Prayer). But we do know for sure that he was sanctified from the womb. The Bible also refers to Job as exceptionally holy (“blameless”):

Job 1:1, 8 There was a man in the land of Uz, whose name was Job; and that man was blameless and upright, one who feared God, and turned away from evil. . . . [8] And the LORD said to Satan, “Have you considered my servant Job, that there is none like him on the earth, a blameless and upright man, who fears God and turns away from evil?” (cf. 2:3) 

Therefore, by analogy and plausibility, we can and may conclude that it is “biblical” to believe in faith that Mary was immaculately conceived. Nothing in the Bible contradicts this belief. It does require faith, of course. God restored to Mary the innocence of Eve before the Fall, and filled her with grace, in order to make her “fit” for the unspeakably sublime, sanctified task of being the Mother of God the Son.