The object of the present work was to investigate: a) The cytotoxic activity of cytophilic antibody in sera from guinea pigs sensitized with the testicular antigen; b) The presence of immune complexes. In vitro cytotoxicity induced by hyperimmune sera or by the cytophilic antibodies eluted from the corresponding macrophages, was demonstrated after incubation with either germinal cells or spermatozoa. Marked cytotoxicity was also observed in vivo when sera or antibodies were injected into the tests, as shown by sequential histologic studies. Circulating immune complexes, as tested by the 125I-C1q binding assay, were not detected in these sera. On the contrary, high levels of circulating immune complexes were found when the polyethyleneglycol precipitation test was used. It can be concluded that the cytotoxic effect induced by cytophilic antibodies of IgG2 nature, may be associated with polyethyleneglycol precipitating immune complexes.