K562 is a human leukemic cell line used as model of hematopoietic differentiation. A variety of differentiation-inducing agents was used in this study, and the expression of surface membrane antigens associated with specific lineages of differentiation and changes in the cytochemistry of the induced cells were monitored. Sodium butyrate, hemin, retinoic acid, dimethyl sulfoxide (DMSO), phorbol myristate acetate (PMA), and interferon induced unique alterations in the binding of monoclonal antibodies specific for erythroid, granulocytic, monocytic, and megakaryocytic lineages. Hemoglobinization, Sudan Black B, glycogen content, nonspecific esterase, alkaline phosphatase, and 5'-nucleotidase staining were also altered. K562 cells were terminally differentiated with PMA to nitroblue tetrazolium-(NBT) positive macrophages. Expression of 3-fucosyl-N-acetyl lactosamine, previously thought to be myeloid specific but found on all early hematopoietic progenitors, was modulated during differentiation to nonmyeloid lineages. Lineage infidelity was noted during functional differentiation along all hematopoietic lineages. The presence of multiple lineage surface markers and cytoplasmic characteristics in leukemic cells is not indicative of lack of potential to differentiate. K562 cells cannot be compared to any normal stage of hematopoietic differentiation, but they do have the capacity to differentiate along erythroid, macrophage, and megakaryocytic lineages.