Antisera raised in rabbits against purified components gp52, p28 and p12 of murine mammary tumor virus (MuMTV) were first characterized by means of immunodiffusion, immunoelectrophoresis and radioimmunoassay. No significant crossreactivity could be detected. The 3 polypeptides were detected by means of these antisera in the cytoplasm of murine mammary tumor cell lines Mm5mt/cl and C3HMT/cl11, as well as in the murine leukemia cell lines GRSL18 and L1210 by means of fixed cell immunofluorescence. The concentration of gp52 in the cytoplasm seems to be considerably lower than that of the 2 other polypeptides. By use of the membrane immunofluorescence assay, only gp52 proved to be localized on the cell surface. Humoral cytotoxicity was only accomplished with anti-gp52 serum. No reaction was found in either fixed-cell or membrane immunofluorescence assay or the cytotoxicity test with the control cell lines EMT-6 and BALB/3T3 infected with Rauscher murine leukemia virus. Chemicals/CAS: Peptides