Previous studies have shown that an antisense phosphorothioate oligonucleotide to the Rel A subunit of NF- kappa B, as well as vitamin E and related antioxidants, significantly enhanced the differentiation of HL-60 leukemia cells when combined with low levels of 1 alpha, 25-dihydroxyvitamin D3 (vitamin D3) an effect accompanied by a marked inhibition of the transcription factor, NF-kappa B. Curcumin, a potent inhibitor of tumor promotion and of tumor cell growth, has also been shown to have antioxidant properties and to inhibit NF-kappa b. to ascertain whether curcumin would also enhance the differentiation of HL-60 leukemia cells produced by vitamin D3, presumably by interfering with NF- kappa B activity, the effects of curcumin on the differentiation of HL-60 cells produced by low levels of vitamin D3 were measured. Curcumin used alone did not produce a significant degree of differentiation of HL-60 cells; however, this agent markedly enhanced the expression of differentiation markers induced by low levels of vitamin D3. Curcumin also increased the differentiation of HL-60 cells when combined with vitamin D analogues (1,25-dihydroxy-16-ene-23-yne vitamin D3 and 1,25-dihydroxy-16-ene vitamin D3) that share the receptor binding properties of vitamin D3, whereas as vitamin D analogue (1,25-dihydroxy-16,23-diene vitamin D3) that caused significant calcium mobilization, but was less effective than vitamin d3 in binding the receptor, did not cause the differentiation of HL-60 cells in the presence or absence of curcumin. Several dietary compounds structurally related to curcumin (i.e., caffeic acid, chlorogenic acid, and ferulic acid) did not increase the differentiation of HL-60 cells produced by vitamin D3. However, the more lipophilic ethyl of ferulic and caffeic acid were capable of inducing the differentiation of HL-60 cells, as well as enhancing the maturation produced by vitamin D3. Curcumin caused a marked reduction in NF-kappa B activity in nuclear extracts of HL-60 cells exposed to this agent in the presence or absence of vitamin D3, supporting the possibility that NF-kappa B may be a factor in the regulation of the state of differentiation of leukemia cells.