Using Lactobacillus fermenti, Lact. arabinosus and Streptococcus faecalis, tests were made of octopine, arcaine, and agmatine, the metabolic products of arginine, as to their arginine activity in arginine-free medium and inhibitory action in medium containing arginine. It was found that these three compounds do not act as arginine substitute or inhibitory in arginine determination. These results, together with the reports of Snell, seem to indicate that it would be possible to determine the content of arginine in the presence of ornithine, citrulline, agmatine, arcaine, and octopine, using Strept. faecalis.