Cell surface markers have identified considerably more heterogeneity within human T, B, and myeloid neoplasms than was evident by standard morphologic and histochemical techniques. Using markers specific for the lineage and state of differentiation, it is now possible to correlate malignant lymphoid and myeloid cells to their normal cellular counterparts. Considering the complexity of the normal hematopoietic system with regard to 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.