A series of 10 cases of hairy cell leukemia (HCL) in native Japanese patients was studied. The clinicopathological features and the phenotype of the leukemic cells were compared with those of HCL in Western countries. The clinical pictures were similar to those in Western countries but some hematological findings were different. The Japanese patients had more marked leukocytosis, less granulocytopenia and thrombocytopenia, and no dry tap on bone marrow aspiration. The phenotype of hairy cells was much the same in both areas. Two kinds of anti-hairy cell sera were raised in rabbits. One (AHS 406) was obtained from rabbits immunized by injecting whole hairy cells. The other (alpha HC-G and alpha HC-M) was obtained by injection of membrane glycoproteins partially purified from hairy cells. The reactivity of the former antiserum (AHS 406) against leukemic cells from a variety of leukemias including HCL and B-cell chronic lymphocytic leukemia was studied by an immunoprecipitation technique. AHS 406 defined three cell surface antigens on HCL, P-30, P-35 and GP-135 (30,000, 35,000 and 135,000 daltons respectively). These three components are expressed only on neoplastic B lymphocytes. Quantitative differences in expression were observed in different types of leukemia, although all three antigens were most strongly expressed in HCL. alpha HC-G showed reactivity similar to that of OKIa-1. alpha HC-M reacted with hairy cells but not with other leukemic cells except mature granulocytes of chronic myelocytic leukemia. It also reacted with most granulocytes and a few mononuclear cells in normal peripheral blood. Preliminary immunochemical studies suggest that alpha HC-M reacts with a glycoprotein with a molecular weight of 26,000.