Characterization of ceramide-effector(s), which includes protein phosphatase 2A (PP2A) is an important prelude to understanding the molecular basis of sphingolipid-mediated biological effects such as cell growth, differentiation and apoptosis. Recently, the existence of a metal-dependent form of PP2A has been reported (Cai et al., 1995). In this study, we investigated the effects of metal ions and chelators on ceramide-activated PP2A (CAPP). Our study demonstrates that at 0.5 mM concentration, Mg 2+ appears to have no significant effect on either basal or ceramide-stimulated phosphatase activities, whereas Ca 2+ stimulated the basal phosphatase activity, but was inhibitory towards CAPP. Moreover, the divalent cations Cr 2+ , Mn 2+ , Fe 2+ , Ni 2+ , Cu 2+ and Zn 2+ were tested and all were found to be inhibitory towards both CAPP and basal phosphatase activities. By contrast, Cs + and Li + had almost no effect on CAPP, although both stimulated basal phosphatase activity. The effects of EDTA and EGTA were tested and it was observed that EDTA decreased CAPP activity in a dose-dependent fashion, but had no effect upon basal phosphatase activity. These results suggest that CAPP is a metal-dependent protein, but, because Ca 2+ inhibitied CAPP and EGTA was much less potent than EDTA in inhibiting CAPP, Ca 2+ is unlikely to be its metal cofactor.