A batch of highly purified phenoxymethylpenicillin has been examined by eight laboratories in seven different countries, and has been assayed against the phenoxymethylpenicillin standard of the Food and Drug Administration of the US Department of Health, Education, and Welfare. The material examined has been established as the International Standard for Phenoxymethylpenicillin, and the International Unit of Phenoxymethylpenicillin is defined as the activity contained in 0.000590 mg of the International Standard.