IgG binding to purified mouse mammary tumor virus (MuMTV) was quantitated by an enzyme-linked immunoassay (ELISA) using sera from patients with breast cancer or benign breast disease, or from healthy age-matched controls. Significantly greater binding (p<0.01) was found in breast-cancer-derived sera than in the other categories. In addition to IgG reactivity, three breast cancer sera also possessed IgA and IgM reactive with MuMTV by the ELISA assay. Only IgG was reactive in the majority of sera while two sera possessed MuMTV binding activity only in the IgM fraction. Both IgG binding and virolysis of MuMTV were greatly reduced by preincubation of sera with MuMTV. The specificity for MuMTV was further explored with IgG of serum from one breast cancer patient. Human antibody reactive with MuMTV was progressively diminished by preincubating the human serum with increasing concentrations of MuMTV but not by incubation with the type-C AKR murine leukemia virus. Preincubation of MuMTV with a breast cancer serum partially blocked the reactivity of gp52 antiserum with the virus. The results suggest that an antigen related to an MuMTV envelope component is expressed in breast cancer.