Inhibitors of the protease of human immunodeficiency virus type 1 (HIV-1) may inhibit cytoplasmic retinoic acid-binding proteins, cytochrome P450 isoforms, as well as P-glycoproteins. These features of the protease inhibitors might enhance the activity of retinoids. To explore this hypothesis, myeloid leukemia cells were cultured with all-trans retinoic acid (ATRA) either alone or in combination with the HIV-1 protease inhibitors indinavir, ritonavir, and saquinavir. Consistent with the hypothesis, the HIV-1 protease inhibitors enhanced the ability of ATRA to inhibit growth and induce differentiation of HL-60 and NB4 myeloid leukemia cells, as measured by expression of CD11b and CD66b cell surface antigens, as well as reduction of nitroblue tetrazolium. Growth of ATRA-resistant UF-1 cells was also inhibited when cultured with the combination of ATRA and indinavir. Moreover, indinavir enhanced the ability of ATRA to induce expression of the myeloid differentiation-related transcription factor C/EBPepsilon messenger RNA in NB4 cells by 9.5-fold. Taken together, the results show that HIV-1 protease inhibitors enhance the antiproliferative and differentiating effects of ATRA on myeloid leukemia cells. An HIV-1 protease inhibitor might be a useful adjuvant with ATRA for patients with acute promyelocytic leukemia and possibly retinoid-resistant cancers.