By an indirect immunofluorescence technique with prolonged serum incubation on murine mammary tumor (MT) slices, 179 of 424 mice examined were found to possess natural serum antibody (antibodies) that reacted with intracytoplasmic A particles (iAp) of MT cells. The immunologic specificity of this antibody was supported by absorption and blocking experiments. Furthermore, a strong similarity was seen between the mouse antibody reaction on various MT and the fluorescence pattern of rabbit anti-iAp antiserum on these tumors. In female mice, incidence and geometric mean titers of the antibody in part were correlated to the spontaneous MT frequency of the mouse strains examined. Some mice of the strains XVII/Bin and CBA/BinfXVII/Bin, hitherto regarded as "free" of the mouse mammary tumor virus (MuMTV), also contained anti-iAp antibody in their sera. In contrast to MuMTV)-producing CBA/Bin micethese animals did not possess detectable spontaneous antibody reacting with MuMTV-B particles. Therefore, hypothetically, the antibody response in these mice might be induced by incomplete MuMTV expression. In the strain CBA/Bin, females 4 months old and older possessed the antibody in significantly higher geometric mean titers when compared to 4-week-old female mice. The history of lactation seemed to have no influence on the titer of antibody. In the comparatively high MT strains CBA/Bin and C3H/Bin, adult (4-month-old) females had the antibody in significantly higher levels when compared to age-matched males.