Acts 9:5-6 as found in the KJV differs from Acts 9:5-6 as found in most modern translations as follows:
KJV: 5 And he said, Who art thou, Lord? And the Lord said, I am Jesus whom thou persecutest: [it is hard for thee to kick against the pricks. 6 And he trembling and astonished said, Lord, what wilt thou have me to do? And the Lord said unto him,] Arise, and go into the city, and it shall be told thee what thou must do.
NRSV: 5 He asked, “Who are you, Lord?” The reply came, “I am Jesus, whom you are persecuting. 6 But get up and enter the city, and you will be told what you are to do.
The part of the KJV verses that I enclosed in brackets above were not in the original texts of Acts, but they were in the Textus Receptus, which was the basis for the KJV's New Testament. On the other hand, "But" (alla in Greek), as found in the NRSV text above, was in the original text but was omitted in the Textus Receptus.
Sometimes students are mislead to believe that the Textus Receptus was the original text (just as there a surprising number of people who believe that Jesus actually spoke in KJV English). The Textus Receptus was simply the Greek text of the New Testament that was based upon Erasmus' haphazardly assembled 1516 Greek New Testament, which itself was primarily based upon two 12th century documents plus some additions from the Latin Vulgate. For a more complete history of the development of the Textus Receptus and its relation to the KJV, browse:
Bruce Metzger, in his invaluable and definitive A Textual Commentary on the Greek New Testament, Second Edition (New York: United Bible Societies , 1994, page 318), comments on Acts 9:5-6:
After diokeis ["persecuting" in NRSV] (and omitting alla ["But" in NRSV] of ver. 6.) the Textus Receptus adds skleron soi pros kentra laktizein. (6) tremon te kai thambon eipe, Kurie, ti me thelis poiesai; kai ho krios pros auton, which is rendered in the AV [Authorized Version, i.e., KJV] as follows: "it is hard for thee to kick against the pricks. (6) And he trembling and astonished said, Lord, what wilt though have me to do? And the Lord said unto him." So far as is known, no Greek witness [manuscript] reads these words at this place; they have been taken from 26:14 and 22:10, and are found here in codices of the Vulgate, with which ith,p syrh with * copG67 substantially agree (all except the Vulgate add after thambon the words epi to gegonoti auto, taken from 3:10). The spurious passage came into the Textus Receptus, when Erasmus translated it from the Latin Vulgate into Greek and inserted it in his first edition of the Greek New Testament (Basel, 1516).
