A new derivative of penicillin, alphaaminobenzyl penicillin, was found to have in vitro activity against both gramnegative and gram-positive bacteria, thus broadening its antimicrobial spectrum beyond that of any previous penicillin compound. The drug was given both orally and intramuscularly and was essentially nontoxic. In a group of 47 infants and children with specific gastroenteritis who received the drug, the results were good inShigelladysentery and pathogenicEscherichia colienteritis. InSalmonellaenteritis the results, albeit less predictably good, were regarded as comparable to those obtained with the present agents of choice.