We have previously reported, using the polymerase chain reaction (PCR), the presence of a 660 bp sequence homologous to the env gene of MMTV in 38% of the human breast cancers studied, but not in normal breasts nor in other tumors or tissues. We have now investigated the presence of MMTV-like LTR sequences in human breast cancer and normal breast tissue. Primers were selected to amplify a 630 bp sequence homologous to MMTV, but not to the endogenous retrovirus HERV-K10. This sequence was detected in 41.5% of the breast cancers and none of the normal breasts. A larger 1.2 kb LTR fragment was also amplified with high homology to MMTV. Finally, a 1.6 kb fragment containing env and LTR sequences was amplified, cloned and sequenced from breast cancer DNA. The human LTRs were highly homologous to MMTV contain enhancer and promoter elements, the glucocorticoid responsive element (GRE) and the superantigen (Sag) sequences. Presence of functional sequences implies involvement in transcriptional regulation, whereas presence of an env-LTR sequence indicates contiguity within the genome of a potential provirus. Their presence in breast cancer DNA, but not in normal tissue, suggest an exogenous origin.