Abstract
The action of 6-thioguanine on the division of PHA-stimulated human lymphocytes in vitro has been investigated and shown to have a definite antimitotic effect which is prevented by adenine but not by guanine or hypoxanthine. It is suggested that in this test system 6-TG exerts its main inhibitory action on phosphoribosylpyrophosphate amidotransferase.