Differentiation induction therapy provides an alternative therapeutic approach for patients with acute myeloid leukaemia (AML) who are either unsuitable for or unresponsive to conventional cytotoxic chemotherapy. The effect of a triple combination of retinoic acid (RA) + 6-thioguanine (6-Th) + hexamethylene bisacetamide (HMBA) on differentiation of blasts from 24 AML patients was studied. Nonadherent mononuclear cells were seeded at a concentration of 5 x 10(5) cells/ml in 24-well tissue culture plates containing RPMI-1640 culture medium with 20% fetal calf serum and 10% 5637-conditioned medium and incubated with 10(-6) M retinoic acid, 1.5 X 10(-6) M 6-thioguanine and/or 2 mM hexamethylene bisacetamide for six days at 37 degrees C in a humidified incubator under 5% CO2. Morphological, cytochemical and functional differentiation into mature cells were induced in blasts from 22 out of the 24 AML patients following exposure to the triple combination of 10(-6) M RA + 1.5 X 10(-6) M 6-Th + 2 mM HMBA in primary culture. These effective results justify a clinical trial of such triple combination for AML patients who are either unsuitable for or unresponsive to conventional cytotoxic chemotherapy.