Mutated ras genes are frequently found in human cancer. However, it has been shown that oncogenic ras inhibits growth of primary cells, through pathways involving p53 and the cell cycle inhibitors p16INK4a and p19ARF. We have analysed the effect of the ectopic expression of the three mammalian ras genes on the proliferation of K562 leukemia cells, which are deficient for p53, p16INK4a, p15INK4b and p19ARF genes. We have found that high expression levels of both wild-type and oncogenic H-, K- and N-ras inhibit the clonogenic growth of K562 cells. Induction of H-rasV12 expression in K562 transfectants retards growth and this effect is accompanied with an increase of p21WAF1 mRNA and protein levels. Furthermore, p21WAF1 promoter is activated potently by oncogenic ras and less pronounced by wild-type ras. This induction is p53-independent since a p21WAF1 promoter devoid of the p53 responsive elements is still activated by Ras. Finally, inhibition of p21WAF1 expression by an antisense construct partially overcomes the growth inhibitory action of oncogenic H-ras. Altogether, these results indicate that the antiproliferative effect of ras in myeloid leukemia cells is associated to the induction of p21WAF1 expression and suggest the existence of p19ARF and p16INK4a-independent pathways for ras-mediated growth inhibition.