The exchange of glycine carboxyl carbon with CO2 catalyzed by the combination of chicken liver glycine decarboxylase (P-protein) and aminomethyl carrier protein (H-protein) was markedly inhibited by various divalent cations, although extents of inhibition by individual metal ions varied considerably. Cu2+ and Zn2+, at 100 microM, inhibited the reaction almost completely, and the inhibitions by Co2+ and Ni2+ were also significant, while Mg2+ and Mn2+ did not appreciably affect the reaction. The inhibition by Zn2+ was competitive with both bicarbonate and H-protein and non-competitive with glycine. Of the two reactions involved in the glycine-CO2 exchange, decarboxylation of glycine yielding the H-protein-bound aminomethyl moiety was not significantly affected by 100 microM Zn2+ or Cu2+, but carboxylation of the H-protein-bound aminomethyl moiety to form glycine was strongly inhibited by either Zn2+ or Cu2+. Various degrees of inhibition of the glycine-CO2 exchange by other divalent metal ions could also be accounted for by the inhibition of the carboxylation step of the exchange reaction. The primary site of the action of divalent metal ions is likely to be not P-protein but H-protein, and the binding of metal ions with the H-protein-bound intermediate of glycine decarboxylation was assumed to account for the observed marked inhibition.