Cytodifferentiation therapy by all-trans-retinoic acid (RA) for acute promyelocytic leukemia patients is encouraging in spite of several limitations preventing better clinical outcomes. Most patients in complete remission induced by RA experience relapse and resist further treatment with RA. This resistance primarily is due to a systemic self-induced catabolism of RA, which interferes with the maintenance of effective plasma levels of RA. In this report we explored the possibility that treatment with combinations of RA and other differentiation agents may induce differentiation at lower RA concentrations, which in turn may produce diminished levels of resistance. We found that although n-butyric acid (BA), tributyrin (TB) (a prodrug of BA), or hexamethylene bisacetamide (HMBA) were inactive as sole agents they potentiated RA-induced differentiation of human acute promyelocytic NB4 cells. A measure of the effectiveness of these combinations was that the concentrations of RA in combination with BA and HMBA inducing half-maximal differentiation were 20- to 40-fold lower than those needed with RA alone. Furthermore, the concentrations of BA and HMBA in these combinations were at achievable plasma levels. Therefore, these combinations may have clinical utility for treatment of a variety of malignancies that are sensitive to RA alone.