Neoplastic populations from 25 cases of B-lymphocytic leukemia (B-LL) were investigated in an attempt to define the stages of maturation arrest represented in this disease and the relationship, if any, to various clinical parameters. Intrinsic to this study was the expression of a number of B-cell antigens defined by monoclonal antibodies. These included antibodies to B1 and B2, both expressed exclusively on B lymphocytes, but with the latter probably restricted to a narrow window of differentiation, BB-1 and LB-1, both markers of activated lymphocytes, and 38.13, a monoclonal antibody raised against and recognizing an epitope expressed on Burkitt lymphoma cells. While the patterns of reactivities were complex, the cases could be classified into various groups on the basis of the phenotypes revealed. These were termed “pre-B,” “early-B,” “intermediate-B,xyc “mature-B,” and “secretory- B” from their patterns of immunoglobulin (Ig) isotype expression and their capacity to secrete Ig in short-term culture. Although B1 was detected on neoplastic cells from all but one case, its intensity of expression varied markedly, being strongest on the “mature-B” and “secretory-B” cases and weakest on the “pre-B” type. The expression of B2 was essentially restricted to the “intermediate-B,” “mature-B,” and “secretory-B” cases and was usually, but not invariably, accompanied by the coexpression of surface IgM and IgD. While expression of BB-1 was restricted to a few cases with mature features, LB-1 was more frequently detected and found predominantly on “intermediate-B” and “mature-Bxyc cases. The 38.13 antibody was found to react weakly with a number of the populations coexpressing IgM and IgD. The export of large amounts of whole Ig was restricted to three “secretory-B” cases all of which were associated with a serum M-component corresponding to the Ig isotype secreted in vitro. In contrast, the secretion of free Ig light chains was a consistent feature of all B-LL, and the amounts detected did not vary considerably between the different types. No correlation between immunologic cell-type and the stage of the disease was apparent. Classification of the cases into “true” B-chronic lymphocytic leukemia and immunocytoma similarly revealed no strict association of these histopathologic entities with any particular phenotypic group, although those cases with the more mature features tended to be immunocytomas. The findings are discussed within the context of normal B-cell differentiation pathways.