The recent finding that retinoic acid induces terminal granulocytic differentiation of the human promyelocytic leukemia cell line, HL-60, prompted an investigation of the sensitivity to this inducer of human myelocytic leukemia cells in primary suspension culture. Of the 21 leukemic specimens, only cells from the two patients with acute promyelocytic leukemia differentiated in response to retinoic acid. After an incubation period of 5–7 days in 1 µM retinoic acid, the cells from these two patients showed extensive morphological and functional maturation. Thus, because it appears that retinoic acid specifically induces granulocytic differentiation of leukemic promyelocytes, this compound may have therapeutic utility in the treatment of acute promyelocytic leukemia.