A microtechnique of CF [complement fixation] could be applied to the study of human BLA [B [bone marrow derived] lymphocyte alloantigens]. These antigens could be detected on PBBL [bone marrow derived-cell enriched suspensions], PBL [peripheral blood lymphocytes], stimulated by PHA [phytohemagglutinin] PWM [pokeweed mitogen] and Con A [concanavalin A], B lymphocytes from CLL [chronic lymphocytic leukemia] and from cultured lymphoid cell lines. The anti-BLA antibodies used were obtained from anti-HLA-A,B sera absorbed on platelets or were acid eluates made from lymphoid cells sensitized by the appropriate immune sera. These reagents made possible the definition of two B lymphocyte specificities (KUB and BOUR).