Immunofluorescence studies using a polyvalent anti-MMTV serum revealed the presence of MMTV antigen(s) on the surface of spleen cells from the low mammary tumor incidence mouse strain BALB/cCrgl. Upon separation of the splenocytes on nylon-wool columns the populations of cells exhibiting positive fluorescence were nylon-adherent. These cells were considered to be B lymphocytes by the following criteria: presence of surface immuno-globulin (Slg), absence of Thy-1 antigens, enhanced responses to B-cell mitogens and decreased reactivity to T-cell mitogens. In order to ascertain the specificity of the cell surface immunofluorescence reaction, blocking studies were performed. Immunofluorescence was blocked by preincubation of the antiserum or with purified MMTV, but not by R-MuLV. Non-specific binding of the anti-MMTV to Fc receptors present on B lymphocytes was ruled out by the use of Fab′2 fragments of IgG from the various sera. Double label immunofluorescent studies with rhodamine-conjugated anti-mouse Slg Fab′2 fragments and rabbit anti-MMTV Fab′2 with fluorescein-conjugated goat anti-rabbit Fab′2 showed the simultaneous appearance of both labels in the nylon-adherent splenocytes. These results suggest the expression of viral polypeptide(s) encoded by an endogenous MMTV on the surface of B-type splenocytes in mice devoid of intact viral particles.