The effects of Ca<2+> and Zn<2+> on 5'-nucleotidase activity in rat liver plasma membranses were investigated. Addition of Ca<2+> (1.0-100 μM) caused a significant decrease of the enzyme activity. The decrease of 5'-nucleotidase activity induced by Ca<2+> (25μM) was completely restored by the presence of a calcium-binding protein (CaBP 5.0 μM). Of the various metals tested, 5'-nucleotidase activity was significantly decreased by the presence of Zn<2+> and Cd<2+>, while Cu<2+>, Co<2+>, Ni<2+> and Mn<2+> did not inhibit the enzyme. In particular, the effect of Zn<2+> was remarkable (decrease of about 60%). A significant decrease of the enzyme activity was also seen at 5.0 μM Zn<2+>, but not at 1.0 μM Zn<2+>. The inhibitory effect of 25μM Zn<2+> on 5'-nucleotidase was not appreciably blocked by the presence of CaBP (2.5 and 5.0 μM). The present data suggest that CaBP uniquely regulates the Ca<2+> effect on 5'-nucleotidase activity in the hepatic plasma membranes. It is proposed that CaBP, which may regulate the Ca<2+> effect on liver cell function, should be called regucalcin.