Bone marrow trephine biopsies from 17 patients with B-chronic lymphocytic leukemia (B-CLL) were studied by immunohistologic techniques in order to investigate the cellular phenotypes of both neoplastic (B-lymphoid) and reactive (T-lymphoid) infiltrates. For this purpose, several heteroantisera and monoclonal antibodies against human Ig isotypes, HLA-DR antigens, and T-cell subpopulations were used in immunofluorescence. The findings were analyzed in relationship to the histologic pattern of involvement, as well as to the immunologic data of cell suspensions from peripheral blood. In all cases, the dominant lymphoid population within the bone marrow infiltrates showed identical phenotypic characteristics of B-CLL cells from the blood (HLA-DR+, mu +, most frequently delta +, kappa +, or lambda +, and weakly RFA-1+). The infiltration by these malignant B cells was diffuse in 5 cases and nodular plus interstitial in 12. The number of T cells (UCHT1+, RFA-1+, mu) was variable (5%-25%) in the different samples, but the values were high when compared to the proportion of T cells in normal bone marrow and in the blood of most patients studied. Furthermore, a clear predominance of T cells exhibiting the inducer phenotype (Leu-3+) was observed in all bone marrow samples, which is in contrast with the findings from peripheral blood, where T cells with the suppressor/cytotoxic phenotype (Leu-2+) were dominant. These data suggest a different blood and tissue distribution of inducer and suppressor/cytotoxic cells in B-CLL, which may have important pathophysiologic significance.