Two continuous-suspension lymphocyte lines were isolated from the spleen and fibrosarcoma of a New Zealand Black female mouse. A C-type virus with a density of 1.16 g x cm(-3), 70S RNA, and RNA-directed DNA polymerase activity was isolated. The virus was infectious for NRK, NZB, (NZB x NZW)F(1), and (NZW x NZW)F(1) embryos, and for BALB/c 3T3 cells, but not for NIH Swiss cells. All cells from established lymphocyte cultures, as well as some embryo cells from New Zealand Black mice, showed karyotypic abnormalities. The possibility of chromosomal mosaicism is suggested.