Phytochemicals present in the genus Allium have potential pharmacological effects, such as antimicrobial, antithrombotic, antitumor, hypolipidaemic and hypoglycemic activities. In this present study, we examined the effects of garlic and onion oils on human promyelocytic leukemia cells, HL-60. Incubation of HL-60 with garlic or onion oil (20 microg/ml) caused a marked suppression of HL-60 proliferation; the suppression was almost identical with those obtained by all-trans-retinoic acid (ATRA) or dimethyl sulfoxide (DMSO) used as positive controls. These oils induced the generation of nitroblue tetrazolium (NBT)-reducing activity, and about 20% of the HL-60 cells became NBT positive. CD11b, another marker of the differentiation of these cells, was also significantly induced by garlic oil or onion oil. The combination of garlic or onion oil with ATRA was more effective than either alone. These data suggest that garlic and onion oils have the ability to induce differentiation of HL-60 cells into those of the granulocytic lineage.