From ten consecutive patients with acute myeloid leukemia leukemic cells were isolated and cultured with and without 10(-6), 10(-7) and 10(-8) M 1.25(OH)2D3 (vit D3), retinoic acid (RA) cytosine-arabinoside (ARA-C) and 1.0, 1.25 and 1.5% dimethyl sulfoxide (DMSO). Maturation was measured with a comprehensive panel of qualitative and quantitative parameters of maturation. Six of those ten leukemias showed significant (p less than 0.01) changes in at least three parameters after exposure to either one of the differentiation inducers. Vit D3 induced maturation in four leukemias, in three of them clearly in monocytic direction. ARA-C showed changes in one leukemia in only three parameters not pointing to either granulocytic or monocytic direction. Maturation in granulocytic direction was observed after exposure to RA in one leukemia. Maturation induction was observed in six out of ten freshly isolated leukemic cells with vit D3 being the most potent inducer of maturation in monocytic direction. The data about inducibility of maturation in freshly isolated human leukemic cells are reviewed and discussed.