Immunophenotypes and Ig gene rearrangements were investigated in 12 patients with a variant form of hairy cell leukemia (HCL) termed HCL-Japanese variant (HCL-J), and in an HCL-J-derived cell line. The leukemic cells of HCL-J characteristically showed the phenotype of CD20+, CD5-, CD10-, CD11c+, CD22+, CD24- and CD25-. Ig light (L) chain was undetected in nine cases, and the remaining four cases expressed kappa chain. Expression of Ig heavy (H) chain was studied in nine cases. In addition to Igkappa+ cases showing expression of predominantly gamma H chain isotype, alpha chain was detected in one case without expression of L chain. Rearranged bands in Ig heavy chain (JH) genes were recognized in all 12 cases tested. Rearranged bands in kappa chain genes and germline configuration in chi chain genes were seen in all three Igkappa+ cases tested. Four of nine cases without expression of L chain had a rearranged chi chain gene. The other three cases had chi chain genes in the germline configuration and rearranged and/or deleted kappa chain genes. In the remaining two cases, no rearrangement in either kappa or chi chain genes was detected. The Ig gene configuration and expression in HCL-J, partially overlapping with those described for immature B cell leukemia, were dissociated from the cytological features and CD20+, membrane CD22+ phenotype characteristic of mature B cells.