Differentiation induction therapy provides an alternative therapeutic approach for patients with acute myeloid leukemia (AML) who are either unsuitable for or unresponsive to conventional cytotoxic chemotherapy. The effect of a triple combination of retinoic acid (RA) + low concentration of cytosine arabinoside (Ara-C) + hexamethylene bisacetamide (HMBA) on differentiation of blasts from 24 AML patients was studied. Non-adherent mononuclear cells were seeded at a concentration of 5 × 105 cells per ml in 24-well tissue culture plates containing RPMI 1640 culture medium with 20 per cent fetal calf serum and 10 per cent 5637-conditioned medium and incubated with 10−6 M retinoic acid, 10−6 M cytosine arabinoside and or 2 mM hexamethylene bisacetamide for six days at 37 °C in a humidified incubator under 5 per cent 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 + 10−6 M Ara-C+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.