The effect of the tumor promoter okadaic acid on cell cycle progression and on vimentin expression in MPC-11 mouse plasmacytoma cells was compared with that of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Cell cycle progression of asynchronously grown MPC-11 cells was inhibited by both agents, but, in contrast to the G1 phase arrest caused by TPA, okadaic acid gave rise to G2/M phase and S phase arrest. This effect of okadaic acid was delayed significantly compared to the TPA-caused arrest. Furthermore, okadaic acid was able to induce vimentin expression to an extent comparable to the TPA response. However, vimentin expression was markedly delayed in okadaic acid-treated relative to TPA-treated cells. Another protein phosphatase inhibitor, calyculin A, also induced cell cycle changes and vimentin expression at concentrations at or above 1 x 10(-9) M. Based on these observations, we suggest an involvement of protein phosphatase 1 (possibly also phosphatase 2A and/or other phosphatases) in both the G2/M cell cycle block and the induction of vimentin expression in MPC-11 cells by okadaic acid.