We found citrulline-containing proteins in mouse peritoneal macrophages undergoing calcium ionophore-induced apoptosis. Such proteins were products of deimination of arginine residues catalyzed by endogenous peptidylarginine deiminase (EC 126.96.36.199) activated by calcium influx. Western blotting analyses of the extract from macrophages incubated with 1 microM ionomycin showed selective deimination of vimentin without detectable degradation. Double immunofluorescence staining of deiminated proteins and vimentin suggested localization of deiminated vimentin around the periphery of round-shaped nucleus, which was thought to be an early morphological sign of apoptosis. The biological implication of vimentin deimination in macrophage apoptosis is discussed.