Lymphocyte function-associated antigen 1 (LFA-1) is a membrane glycoprotein involved in a wide variety of functional activities mediated by human leukocytes. For example, expression of LFA-1 is required on immunoregulatory lymphocytes for functional activity. The appearance of LFA-1 in lymphocyte ontogeny was studied using leukemic cells as probes of distinct maturation stages. LFA-1 was expressed on the neoplastic cells of all mature T-cell neoplasias including T-prolymphocytic leukemia cells but not on immature T-cell blasts in acute lymphoblastic leukemia. The expanded T gamma-cell population in T gamma-lymphocytosis patients expressed very high amounts of LFA-1 compared to normal T cells. The malignant cells in B-chronic lymphocytic leukemia and common acute lymphoblastic leukemia were negative in all cases tested except one. In all B-prolymphocytic leukemia's and some of the B-Non Hodgkin's lymphoma patients the neoplastic cells expressed LFA-1. The plasma cells in two patients with plasmacytoma were found to lack LFA-1. It is concluded that LFA-1 is expressed on mature immunocompetent lymphocytes and their neoplastic counterparts.