SummaryA single injection of crystalline penicillin G potassium administered subcutaneously or intraperitoneally is toxic for hamsters in proportion to amount administered. Toxicity is of the delayed type, the vast majority of deaths occurring on 3rd, 4th and 5th days following injection. This parallels the reported experience with guinea pigs and is in contrast to Swiss mice who easily tolerate very large doses of this antibiotic. In the light of these findings, reevaluation of experimental data concerned with Syrian hamsters either treated with or injected with material sterilized with penicillin would seem to be indicated.