When exposed to sodium butyrate (NaBut), exponentially growing cells accumulate in G1 and G2 phases of the cell cycle. In the human breast cancer cell line MDA-MB-231, an arrest in G2 phase was observed when the cells were released from hydroxyurea block (G1/S interface) in the presence of NaBut. The inhibition of G2 progression was correlated with increased contents both of total p21(Waf1) and of p21(Waf1) associated with cyclin A and with an inhibition of cyclin A- and B1-associated histone H1 kinase activities measured in cell lysates, as well as with dephosphorylation of the RB protein. A decrease in the cell contents of cyclins A and B1 was also observed but this decrease was preceded by p21(Waf1) accumulation. When NaBut was removed from the culture medium of cells blocked in G2 phase, p21(Waf1) level decreased and, instead of proceeding to mitosis, these cells resumed a progression toward DNA rereplication. These results suggest that the induction of p21(Waf1) by NaBut leads to the inhibition of the sequential activation of cyclin A- and B1-dependent kinases in this cell line, resulting in the inhibition of G2 progression and rendering the cells competent for a new cell division cycle.