Arsenic trioxide (As(2)O(3)) has recently been successfully used to treat all trans retinoic acid (ATRA) resistant relapsing acute promyelocytic leukemia. However, its molecular mechanisms of action are poorly understood. In the present study, we used the human leukemia (HL-60) cell line as a test model to study the cellular and molecular mechanisms of anti-cancer properties of As(2)O(3). We hypothesized that As(2)O(3)-induced expression of stress genes and related proteins may play a role in the cellular and molecular events leading to cell cycle modulation in leukemic cells. To test this hypothesis, we performed Western blot analysis to assess the expression of specific cellular response proteins including p53, c-fos, RARE, Cyclin A, and Cyclin D1. Densitometric analysis was performed to determine the relative abundance of these proteins. Western Blot and densitometric analyses demonstrated a strong dose-response relationship with regard to p53 and RARE expression within the dose-range of 0-8 microg/ml. Expression of c-fos was slightly up-regulated at 2 microg/ml, and down-regulated within the dose-range of 4-8 microg/ml. A statistically significant down-regulation of this protein was detected at the 6 and 8 microg/ml dose levels. No statistically significant differences (p > 0.05) in Cyclin D1 expression was found between As(2)O(3)-treated cells and the control. Cyclin A expression in As(2)O(3)-treated HL-60 cells was up-regulated at 6 microg/ml, suggesting that it is required for S phase and passage through G(2) phase in cell cycle progression. Taken together, these results indicate that As(2)O(3) has the potential to induce cell cycle arrest through activation of the 53-kDa tumor suppressor protein and repression of the c-fos transcription factor. Up-regulation of RARE by As(2)O(3) indicates that its cytotoxicity may be mediated through interaction/binding with the retinoic acid receptor, and subsequent inhibition of growth and differentiation.