The distribution of antigenic determinants on a cellular level in serous ovarian neoplasms was evaluated using polyclonal and two monoclonal antibodies (OC 125 and 10B). The expression of antigens was estimated by an immunofluorescence test on each cell fraction isolated by density centrifugation from cystic fluids of individual malignant and benign ovarian tumors, taking into account the density and cytomorphologic features of cell subpopulations. It was found that the studied antibodies recognized different antigenic determinants. Significant immunologic heterogeneity of cells among and within individual tumors was shown. Our studies show the value of isolated cell subpopulations for comparing the reactivity of different antibodies and estimating their immunodiagnostic potency.