This study examined the defensive ability of human dental pulp against H2O2 in healthy and reversible and irreversible pulpitis tissues through determination of catalase activity by spectrophotometric methods. Thirty-five systemically healthy patients were donors of the pulp tissue, and pulp conditions were assessed using clinical and X-ray evaluations. Catalase activity was 1.61 +/- 0.23 U mg(-1) protein in the healthy tissues, 2.99 +/- 0.45 U mg(-1) protein in the reversible pulpitis tissues, and 2.44 +/- 467 mU mg(-1) protein in the irreversible pulpitis tissues. All differences between the groups were statistically significant. These results point to a role for catalase during dental pulp inflammation in humans, and therefore demonstrate an inherent biological defense system against reactive oxidants in human dental pulp.