Proteus rettgeri has been found to contain two separable 1-amino acid oxidases. Both enzymes are particulate in nature, neither being ribosomal bound. One of these enzymes appears to have broad specificity, being active toward monoaminomonocarboxylic, imino, aromatic, sulfur-containing, and beta-hydroxyamino acids. The other enzyme has more limited specificity, catalyzing the oxidative deamination of the basic amino acids and citrulline. The affinity of this oxidase for the various substrates at pH 7.6 in decreasing order is arginine, histidine, ornithine, citrulline, and lysine. This enzyme has a particularly high affinity for arginine (Km equal to 0.27 mM), and anomalous kinetics are observed with increasing substrate concentrations. When concentrations of arginine greater than 1.0mM were added to the reaction containing histidine, imidazole pyruvate formation was completely inhibited.