All-trans-retinoic acid (RA), sodium n-butyrate (NaB), hexamethylene bisacetamide (HMBA), and dimethyl sulfoxide (DMSO) induce differentiation of the human acute myeloid leukemia cell line HL60. In the clinic, RA, NaB, or HMBA induce complete or partial remissions. However, the achievement and maintenance of effective plasma concentrations and toxicity have been problems. These difficulties led us to study the interaction of RA with these inducers. We found that combinations of RA with either NaB, HMBA, or DMSO synergistically induced terminal differentiation of HL60. A measure of the effectiveness of these combinations was that the doses of NaB, HMBA, and DMSO required alone to induce half-maximal differentiation were decreased about 4-fold in combination with normal plasma concentrations of about 30 nm RA. RA or NaB alone did not enhance the growth of HL60 cells. In contrast, HMBA or DMSO alone increased growth of HL60 cells even at concentrations that did not induce differentiation. The addition of RA reduced the promotion of growth and increased the extent of terminal differentiation seen with HMBA and DMSO alone. These data suggest that treatment of some malignancies with combinations of RA with HMBA or NaB may maintain differentiation-inducing effects and decrease the problems associated with the achievement and maintenance of effective plasma concentrations as single agents.