An increase in the number of revertant colonies in the Ames test is generally taken as a strong indication of mutagenic activity of a test compound. However, irrelevant positive findings may constitute a major problem in regulatory drug testing. In this study, mixtures containing only amino acids such as glycine, lysine, arginine and isoleucine, routinely used as peptide preservatives in polypeptide pharmaceutical products, were investigated for mutagenesis in the Ames Salmonella typhimurium test. The results demonstrated that in the presence of metabolic activation, all the solutions containing arginine induced an increase in the number of revertant colonies in strains TA98, TA100 and TA1535 compared with the solvent control. More specifically, for strain TA98, all arginine doses tested, i.e. from 0.4 to 8mg/plate induced a statistically significant increase in the number of revertants. This increase was biologically significant from 1.2 to 8mg/plate. For strain TA100, the five highest test doses, i.e. from 1.2 to 8mg/plate, induced statistically and biologically significant increases in the number of revertants. A statistically significant increase in colony number was also observed in strain TA1535, but only at the maximal test dose of 8mg/plate arginine. These increases were observed with arginine from two different sources, suggesting that the observed effect would not be due to the presence of potential impurities in the type of arginine used. Our findings show that a functional metabolic activation system was required to induce an increase in the number of colonies. The presence of vitamin C inhibited the arginine-induced increase in the number of revertant colonies in S. typhimurium strain TA98, suggesting a potential involvement of oxidative stress.