Leukocyte antigens of approximate molecular weight 200,000 daltons have been described in mouse, rat, and man. We describe here the reactivities of a monoclonal antibody, GAP 8.3, which identified such an antigen on human leukocytes. We found the leukocyte antigen H-T200 on T and B lymphocytes, granulocytes, monocytes, and platelets, but not on erythrocytes or nonhematopoetically derived cells. Resting and activated T cells had more antigen on their surfaces than did resting B lymphocytes and EBV-transformed B cells, respectively. The leukocyte antigen was detected on approximately 75% of bone marrow cells; cells of the erythroid series comprised the negative population. The GAP 8.3 antibody and its F(ab')2 fragments had no effect on in vitro stimulation of peripheral blood cells by mitogens or allogeneic cells. Antigen isolated from T cell lines had a higher electrophoretic mobility than did antigen from B cell lines; antigen from the myeloid line U937 comigrated with that from B cell lines. In addition, we detected very small but reproducible differences in the electrophoretic mobility of the antigen on two T cell lines.