The use of MAbs directed against B-cell markers has identified considerably more heterogeneity within B-cell neoplasms than was evident by standard morphologic and histochemical techniques. Using markers specific for lineage and state of differentiation, it is possible to correlate malignant B cells to their normal cellular counterparts. Considering the complexity of normal B-cell ontogeny, differentiation, and function, it is not surprising that these malignancies reflect this diversity. Hopefully, with increasing characterization of the normal function of cell surface molecules, as well as the subpopulations of normal cells to which these malignancies correspond, we will have a better understanding of the biologic and clinical behavior of these malignancies.