Studies on the role of purines as cofactors in the enzymatic reduction of tetrazolium salts by monoamines have led to the following results: (1) With whole rat liver extracts as the source of enzymes, several purines exhibit cofactor activity either as the free base or as the corresponding riboside and ribotide derivatives. (2) In contrast to this, mitochondrial material from rat liver is active only if adenylic acid or one of several ribotidic derivatives containing an adenylyl or similar moiety is used as cofactor. (3) Mitochondrial material utilizes hypoxanthine as cofactor for the amine/tetrazolium system only in combination with the supernatant obtained by centrifugation of tissue homogenates at 20,000 g. The additional factor present in this supernatant portion is heat-labile and nondialyzable. The possibility that this additional factor is an enzyme or enzymes converting the free base to the ribotide is discussed.Inhibition studies have revealed that the amine/tetrazolium enzyme system is sensitive to several metal-binding agents, but no direct evidence for the role of a metal in the enzymatic reaction could be obtained. It was also found that nicotinamide and adenine, neither of which exhibits cofactor activity, are potent inhibitors of the enzyme system studied.