To determine whether protein phosphorylation and dephosphorylation can affect apoptosis in osteoblastic cells, we examined the effects of okadaic acid (OA) and calyculin A (CA) on cultured human osteoblastic cells Saos-2 and MG63, and mouse osteoblastic MC3T3-E1 cells. After reaching confluence, these cells were exposed to varying concentrations of OA or CA. OA and CA induced cell death in all three cell lines in a dose- and time-dependent manner. Marked nuclear condensation and fragmentation of chromatin were also observed in these cells by using the Hoechst 33342 stain. DNA ladder formation, a hallmark of apoptosis, was detected in Saos-2 and MG63 cells, but not in MC3T3-E1 cells by treatment of OA or CA. In the Saos-2 cells, OA- and CA-induced DNA ladder formation was dose-dependent with maximal effect at concentrations of 10 and 2 nM, respectively, and was time-dependent from 14 to 48 h. DNA ladder formation in response to OA and CA was revealed by using conventional ethidium bromide staining of electrophoresed DNA without using autoradiography. Beyond the maximal effects at the respective concentrations, however, cell death did not indicate DNA laddering, suggesting that phosphatase activity may be required for ladder formation. Our results indicate that apoptosis in the cultured osteoblastic cells is induced by moderate inhibition of PP-1 or PP-2A based on the known selectivity of okadaic acid and of calyculin A.