The nucleotide sequence of the long terminal repeat (LTR) of bovine leukemia virus, a unique oncogenic retrovirus of cattle, was determined. The LTR consisted of 530 base pairs (bp) with an inverted repeat of 6 bp at its 5' and 3' ends, flanked by a direct repeat of 6 bp of host cell origin. A tRNAPro binding site for minus-strand DNA synthesis followed the 5' LTR. The U3 region contained putative transcriptional promoters, "CAT" box and "TATA" box, but they had peculiar sequences (C-C-A-A-C-T and G-A-T-A-A-A-T). The U3 region also contained a potential enhancer element, whose sequence partially resembled those of other viral and cellular, especially of immunoglobulin, enhancers. The most striking structural feature of the LTR was an exceptionally long R region (228 bp), which separated a poly(A) addition signal (A-A-T-A-A-A) from a poly(A) site as far apart as 260 bp. The long R region was suggested to form a large stable hairpin structure on a nascent RNA chain, making the two transcription termination signals close together and thus ensuring normal termination of the chain. This structural feature of the bovine leukemia virus LTR was analogous to that of human T-cell leukemia virus LTR and, in fact, slight sequence homology (at most 50%) was observed between the R regions of these two retroviruses, indicating their evolutionary relationship. The unique structural feature of bovine leukemia virus and human T-cell leukemia virus LTRs may thus bear some relation to the biological features commonly shared by these retroviruses.