Growth inhibition by adenine or adenosine and its reversal by other purines were quantitatively different in the fourStaphylococcus aureusstrains we have studied. After 11 hours, 0.25 μmole/ml adenine or adenosine reduced growth by 6 and 4%, respectively, in the coagulase-negative SA-13 strain, by 35 and 35% in the tryptophan-dependent Smith strain, by 68 and 98% in a Wood 46 thymineless mutant strain, and by 40 and 37% in the parent Wood 46 strain. Normal growth in the presence of adenine or adenosine was restored in most cases by guanine, guanosine, hypoxanthine, inosine, and inosinic acid. However, for the thymineless mutant strain, guanine and inosine were much less effective in this respect in the presence of adenine, and none of these purines could reverse the inhibitory effect of adenosine. Purine analogs were mostly ineffective in inhibiting or in restoring growth. Our observations indicate that some patterns of purine biosynthesis and utilization inStaphylococcus aureusare different from those already known in other bacterial species. Adenine or adenosine had a greater inhibitory effect on the growth of coagulase-positive strains than on that of the coagulase-negative strain.