Abstract All- trans-retinoic acid (RA) induces differentiation of acute promyelocytic leukemia cells both in vitroand in vivoand is an alternative to cytotoxic chemotherapy in the treatment of acute promyelocytic leukemia. However, despite a complete remission rate of about 90%, most patients relapse and are resistant to further treatment with RA. This resistance primarily is due to an increased systemic catabolism of RA. In this study we examined the catabolism of RA by the human acute promyelocytic leukemia cell line NB4 and the human myeloid leukemia cell line HL60. NB4 cells converted RA to 4-hydroxy-RA, 4-oxo-RA and more polar unidentified retinoids at a much greater rate than HL60 cells. Exposure of NB4 cells to RA induced RA catabolism. We found that 4-hydroxyphenylretinamide (4-HPR) inhibited the catabolism of RA. This inhibition was dose-dependent and greater, on a molar basis, than the inhibition seen with the cytochrome P450 inhibitor, ketoconazole, or two synthetic retinoids, Ch55 and Am80. 4-HPR alone was a poor inducer of differentiation of NB4 cells. However, it markedly enhanced RA-induced differentiation and increased the level of retinoylation, the covalent binding of RA to proteins. These results suggest some retinoid analogs, including 4-HPR, may have clinical utility because of their ability to increase the biological half-life of RA.