All-trans retinoic acid induces leukemic cells from patients with acute promyelocytic leukemia (M3) to differentiate in vitro to mature granulocytes which express the CD15 antigen and are capable of respiratory burst function. Of 35 M3 samples, only one failed to respond. In eight cases, we compared the efficacy of two naturally occurring isomers of retinoic acid, all-trans RA and 13-cis RA. Both isomers induce maximal differentiation at 10(-6) mol/L. The maximal response was maintained at 10(-7) mol/L for the all-trans but not for the 13-cis RA. We also observed that the metabolites 4-oxo-all-trans and 4-oxo-13-cis were effective at 10(-6) mol/L. This 1 order of magnitude difference in the in vitro differentiating potencies of all- trans RA and 13-cis RA in the blasts of promyelocytic leukemias predicts a difference in the clinical efficacy of the two drugs.