Class I antigen expression by ovarian epithelial neoplasms was determined by flow cytometric analysis and an immunoperoxidase technique for each specimen. The numbers of class I positive tumors determined by the methods were compared. The more subjective immunohistochemical analysis and the more objective flow cytometric technique revealed similar results as long as strict criteria for the interpretation of results was applied. Most of the tumor specimens revealed a homogeneous Gaussian distribution of green fluorescence, class I antigen expression, by flow cytometry. There were two specimens that exhibited a less than characteristic type of membrane staining. The antigen-antibody reaction product was expressed in the extracellular matrix, as well as on the cell membrane of certain cells. This may represent a defect in antigen expression and, if so, might alter the immune response to these tumors.