Fifty-two human breast tumors were screened for the presence of DNA homology to mouse mammary tumor virus (MMTV) using molecularly cloned MMTV proviral genomic DNA probes and dot-blot hybridization. Seven patients were found to contain an entire provirus (gag, pol, env, and LTR positive at high stringency). Fifty percent (5/10) of patients having a first degree relative with breast carcinoma were found to have DNA homology to the gag-pol portion of the MMTV genome when hybridization and washing was performed at moderate (56C) stringency. Thirty-nine percent (7/18) of patients with any positive family history and 23 percent (8/34) of patients with a negative family history demonstrated homology under these parameters. Of the patients positive for gag-pol at moderate stringency, fewer had taken exogenous hormones than the sample group (20 percent vs 52 percent), more were parous (93 percent vs 68 percent), estrogen receptor positive (69 percent vs 48 percent), and male (13 percent vs 4 percent). At higher stringency (62C) no correlation to family history, hormone use or sex was detected, but positivity was noted among estrogen and progesterone receptor positive patients (67 percent vs 48 percent). Under lower stringency wash conditions, mismatched MMTV-related sequences are identified suggesting the existence of an endogenous gene with partial homology to MMTV. High stringency hybridization may identify a related retrovirus with significant homology to MMTV.