OBJECTIVE To examine the catalase activity in clinically healthy and symptomatic human dental pulps to verify if an active defence system against oxidizing agents is present as a response to bacterial invasion. METHODS Thirty-three systemically healthy patients, 18 females and 15 males (ages: 11.0-25.9 years; mean 18.8 +/- 3.6), were the source of the pulp tissue. The condition of the pulps was assessed using clinical and radiographic evaluations. The specimens were recovered by longitudinally grooving and splitting the teeth (if extracted) or during endodontic treatment, and were matched for age and sex between the healthy and inflamed specimen groups. Catalase activity was determined through spectrophotometric methods. RESULTS Enzymatic activity was 1126 +/- 343 and 3074 +/- 698 mU mL(-1) x mg of total protein in the healthy and inflamed pulp tissue specimens, respectively. This difference was statistically significant (P<0.01). CONCLUSIONS These results indicate a role for catalase during dental pulp inflammation in humans, and may represent an inherent biological defence system against reactive oxidants of this tissue.