AbstractA variant of the hypoxanthine‐guanine phosphoribosyltransferase deficient, and adenine phosphoribosyltransferase deficient mouse A9 cell line has been obtained by selecting cells which are resistant to 6‐azauridine. These cells are not only resistant to 6‐azauridine (5 × 10−4M), but also to adenosine (10−3M). Resistance persists indefinitely even in the absence of both compounds. The resistant cells are killed by 5‐fluorouridine (10−6M), indicating that the part of the salvage pathway for pyrimidine ribonucleotide biosynthesis which is relevant to the action of 6‐azauridine is intact. The heritable change producing concurrent resistance to 6‐azauridine and adenosine probably involves thede novopyrimidine bi
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