Carnosic acid, the polyphenolic diterpene derived from rosemary, is a strong dietary antioxidant that exhibits antimutagenic properties in bacteria and anticarcinogenic activity in various cell and animal models. In the present study, we show that carnosic acid (2.5-10 microM) inhibits proliferation of HL-60 and U937 human myeloid leukemia cells (half-maximal inhibitory concentration = 6-7 microM) without induction of apoptotic or necrotic cell death. Growth arrest occurred concomitantly with a transient cell cycle block in the G1 phase, which was accompanied by an increase in the immunodetectable levels of the universal cyclin-dependent kinase inhibitors p21WAFI and p27Kipl. Carnosic acid caused only a marginal induction of differentiation, as monitored by the capacity to generate superoxide radicals and the expression of cell surface antigens (CD11b and CD14) and receptors for the chemotactic peptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine. However, at low concentrations, this polyphenol substantially augmented (100- to 1,000-fold) the differentiating effects of 1,25-dihydroxyvitamin D3 and all-trans retinoic acid. Furthermore, such combinations of carnosic acid and any of these differentiation inducers synergistically inhibited proliferation and cell cycle progression. These results indicate that carnosic acid is capable of antiproliferative action in leukemic cells and can cooperate with other natural anticancer compounds in growth-inhibitory and differentiating effects.