We have studied the relationship betwen purine salvage enzymes, 6-mercaptopurine resistance, and the purR phenotype in E. coli. Mutants resistant to 6-mercaptopurine were found to have defects in HPRT, the purR repressor, or in both. Analysis of these mutants led to the isolation of a hypoxanthine phosphoribosyl transferase-guanine phosphoribosyl transferase double mutant (hpt- gpt-) that is extremely sensitive to adenine. Two classes of adenine resistant mutants were isolated from this strain. The first class was deficient in APRT (apt-) while the second class represented purine regulatory mutants (purR-). There is thus selection for the purR phenotype in a hpt- gpt- background.