We raised a murine monoclonal antibody, OG-3, which reacts with human granulosa cells. Immunohistologically, OG-3 antigen was weakly expressed on the granulosa cells of some growing and atretic follicles, but not on those of preovulatory follicles. After ovulation, the antigen expression rapidly increased on granulosa cells during corpus luteum formation. The antigen expression on granulosa/large luteal cells decreased in the mid-luteal phase, but increased again in the late luteal phase. In early pregnancy, OG-3 antigen expression on large luteal cells increased after 7 wk of gestation. The OG-3 antigen distribution in various organs resembled that of human leukocyte antigen (HLA) class II molecules. An HLA-class II-positive human B cell line (AKIBA) and a murine L-cell transfectant expressing HLA-DR antigen were positive for OG-3 antigen, whereas an HLA-class II-negative human T-cell line and L-cell transfectants expressing HLA-DP and DQ antigens were negative. The molecular mass of OG-3 antigen purified from AKIBA cells was 32-35 kDa. The staining profiles in ovaries with anti-HLA-DR or anti-HLA-class II antibodies were similar to that with OG-3. These results indicate that OG-3 antigen is identical to HLA-DR, and that HLA-DR is a differentiation antigen for human granulosa cells.