The 3' terminal region of the Prague strain of Rous sarcoma virus (PrRSV) contains at least three distinct domains that comprise two functional enhancer elements. Two of these domains (designated B and C) are found in the U3 region of the 3' long terminal repeat (LTR) while the third (designated A) is located in the sequences immediately preceding the LTR termed XSR sequences. Combinations of adjacent domains [e.g., (A + B or B + C)] are capable of activating the expression of the SV40 early promoter (21 bp repeats and TATA box) coupled to coding sequences from the prokaryotic gene chloramphenicol acetyltransferase (CAT) while a single domain is inactive. Furthermore, duplication or triplication of the central domain B restores activity. The related, Schmidt-Ruppin, strain of RSV, contains an almost identical 3' LTR element, but differs in the enhancer sequences immediately preceding the 3' LTR. A model is presented in which the sequence differences may contribute to the difference in disease spectrum of transformation defective (td) variants of these viruses.