3-Aminobenzamide is commonly assumed to be a specific inhibitor of poly(ADP-ribose) polymerase. However, it also inhibits de novo synthesis of DNA purines at concentrations of 5 mM and above. A mutant cell line that lacks the ability to incorporate exogenous purines is therefore exceptionally sensitive to the toxic effects of 3-aminobenzamide. This finding has implications for the underlying mechanism of sensitivity to 3-aminobenzamide recently reported in a cell line from an immunodeficient patient, and also implies that many studies using high concentrations of 3-aminobenzamide may not demonstrate a role for poly(ADP-ribose) because of artifacts arising from disturbance of intracellular purine metabolism.