We investigated the surface expression of leukocyte differentiation antigens and the Ig heavy-chain variable region (VH) gene family use in leukemic cells from 26 Japanese patients with chronic B-cell leukemias with special reference to CD5 antigen expression. CD5 was expressed on leukemic cells in 21 of 26 cases (CD5+) but not in 5 cases (CD5-). Myelomonocytic marker, CD13 antigen was expressed on the leukemic cells in all 5 CD5- cases but in none of CD5+ cases. Leukemic cells in CD5- cases also expressed CD11b antigen more frequently than those in CD5+ cases (80% v 11%; P < .01). Another myeloid marker, CD33, was expressed neither on CD5+ nor CD5- leukemic cells. CD22, a restricted B-cell marker, was expressed more frequently on CD5- leukemic cells than on CD5+ leukemic cells (80% v 33%; P < .05). Another restricted B-cell activation marker, CD23, was expressed at similar frequency in both the CD5+ and CD5- groups (67% v 60%). Although CD45RA was expressed on the majority of leukemic B cells, the CD45RA expression level was significantly higher among CD5- cases than CD5+ cases (P < .01). In the analysis of VH gene expressed in chronic B-cell leukemias by polymerase chain reaction amplification, CD5+ cases preferentially used VH4 family members (48%; 10 of 21). CD5- cases, on the other hand, mainly used VH3 family (80%; 4 of 5). Thus, from our present observation of an albeit limited patient population, we have found an association between VH gene family use and CD5 antigen expression in chronic B-cell leukemias. We have also shown the differential expression of myelomonocytic markers in the CD5+ and CD5- chronic B-cell leukemias. These result are in agreement with previous suggestions that CD5 positivity is the hallmark for distinct clinical entity commonly referred to in the literature as chronic lymphocytic leukemia.