During an attempt to develop an Amethopterin-resistant strain of L. casei, a variant was isolated which required thymine or thymidine in addition to pteroylglutamic acid. The normal growth of the variant in purine-f ree medium containing pteroylglutamic acid and thymine indicated that there was no defect in the folic-catalyzed reactions involved in the biosynthesis of purines. This requirement for thymine, an important product of biosynthetic reactions involving folic acid coenzymes, directs attention to the role of products of drug-inhibited reactions in the selection and metabolism of drug-resistant cells.