Glycine decarboxylase, tentatively called P-protein and considered a constituent of the glycine cleavage system, was purified to apparent homogeneity from chicken liver mitochondria. P-protein is a homodimer having a Mr = approximately 200,000 and consisting of identical subunits with Mr = approximately 100,000. Each subunit appears to contain an equimolar pyridoxal 5'-phosphate which is bound to the protein, possibly through a protonated aldimine linkage. The isoelectric point of P-protein was 7.2. P-protein could bind glycine, showing a Kd of 33 mM for it, and could catalyze glycine decarboxylation even though the rate of decarboxylation catalyzed by P-protein alone was extremely low. The product of glycine decarboxylation was methylamine and the Km for glycine. Methylamine could bind to P-protein, giving a Kd value of 63 mM, and it inhibited the glycine decarboxylation. P-protein alone could also slightly catalyze the exchange of carboxyl carbon of glycine with CO2 and the exchange appeared to obey a ping-pong mechanism. Both glycine decarboxylation and the glycine-CO2 exchange catalyzed by P-protein were stimulated 100-fold or more by the addition of lipoic acid, which is a functional group of H-protein. We may define P-protein as glycine decarboxylase although P-protein alone exhibits only very low catalytic activities.