We investigated the effects of two sex steroids (17beta estradiol and testosterone) on five human leukemia cell lines. We observed a statistically significant inhibition of proliferation, dose and time dependent, of the human monoblastic leukemia cell line U937. This inhibition was associated with a dose dependent decrease in the number of CFU-blasts in clonogenic cultures. Cytostatic effect was obtained with doses of 5 microM for estrogen and 10 microM for androgen and was not due to a non-specific cytotoxic effect, some cell viability remained high (> 90%) even after 6 days of incubation. More accurately, we demonstrated that growth inhibition was associated with a cell cycle arrest, U937 cells accumulating in G2/M phase. This blockade was dose related with a maximum number of cells accumulating at day 4. Sensitivity of these cells to an S-phase specific agent (hydroxyurea) was not increased, suggesting that these cells were blocked in G2/M and did not undergo mitosis. Expression in U937 cells of high affinity nuclear receptors for estrogen and androgen was negative which was in favour of a type II estrogen binding site, mediated mechanism. Moreover, a small fraction of these cells underwent apoptosis or differentiation with about 12% apoptotic cells and a significant increase (more than 30%) of two myelomonocytic markers (CD13 and CD64). These results demonstrate that the proliferation of some leukemic cells may be inhibited by micromolar concentrations of sex steroids, independently of nuclear receptor expression. The main mechanism seems to be a block in cell cycle associated with modulation of apoptosis and differentiation. It provided additional evidence for the potential value of sex steroids and their analogues in the treatment of leukemias.