Applying an indirect immunofluorescence assay with prolonged serum incubation on frozen-cut mouse mammary tumor slices, antibodies are detectable in human sera which react with viral inclusion bodies consisting of intracytoplasmic A particles (iAp). These iAp are known to represent intracellular entities of the mouse mammary tumor virus (MuMTV), mainly related to MuMTV core constituents. In 1,449 women studied, the antibody incidence is partially correlated with proliferating changes of the mammary gland. However, epidemiological data obtained thus far have clearly shown that the detection of antibody is not useful in breast cancer diagnosis (antibody incidence in breast cancer patients 22.8%, in lactating women 20.6%, in controls 5.9%). Studying antibody reagibility to particular iAp polypeptides by means of a radioimmunoprecipitation (RIP) technique, there has been preliminary evidence that human serum factors react with more than one mouse virus protein. The most prominent reactivity was seen to be directed to the 14,000 dalton protein (Ap14). The detection of serum antibody activity is considered as being of important indicative value in that it reflects the existence of MuMTV-related antigens in man.