Accumulation of ribotide of 5-amino-4-imidazole carboxamide (AICAR), excreted in the riboside form by a purine-requiring mutant of Escherichia coli, was inhibited by a number of structural analogs of purines. The most potent inhibitors were 6-thioguanine, 6-mercaptopurine, and 2,6-diaminopurine. The type of inhibition obtained suggests that the antimetabolites sufficiently resemble natural purines in structure to act as feedback inhibitors in biosynthetic control.