Previously we reported that HMCAg related to gp52, env product of MMTV, was a specific marker of human breast cancer (BC). This antigen was expressed not only in BC tissue and patients' sera, but in peripheral T- and B-cells. HMCAg was found in 21.4% of healthy donors, but only in the fraction rich in B-cells. We searched for env-homologous sequences in BC patients by Northern and Southern dot-blot hybridization of plasmid clones containing env MMTV with total RNA and DNA of peripheral blood lymphocytes of BC patients, gynecological patients with hormone-dependent tumors, and donors. Transcription of DNA sequences related to env MMTV was revealed in 91.3% of BC patients, 22.2% gynecological patients, and 27.2% donors. In all cases except 1 donor and 1 gynecological patient this transcription correlated with HMCAg expression in lymphoid cells from the same subjects detected by indirect immunofluorescence with anti-gp52 MMTV rabbit serum. Analysis of in vitro translation products with poly(A) RNA as a template and immunoblotting method showed that at least one of the polypeptides formed corresponded to HMCAg by molecular weight and immunological reactivity, which fact appears to explain the correlation described above.