A subchronic 90-d study was conducted with chlorinated and nonchlorinated humic acids using male Sprague-Dawley rats. Body weight gain, terminal organ and body weights, food and fluid consumption, clinical chemistries, hematological parameters, and urinalyses were determined for all animals. Selected organs were examined microscopically. Significant findings were confined to those rats given the high dose of chlorinated humic acid (1.0 g/l total organic carbon). The terminal body weight and average weekly body weight gain were significantly lower (p less than 0.05) in the high-dose group as compared to the distilled-water control group. This difference can be partially explained by a 16% lower daily fluid consumption. The average weight of the kidneys was significantly higher in the 1.0-g/l chlorinated humic group as compared to distilled-water controls. Hematological parameters and clinical chemistry values were normal in all treatment groups. The most significant finding was the increased incidence and severity of hematuria in the 1.0-g/l chlorinated humic acid group. A thorough histopathological examination of the entire urinary tract indicated that the most likely cause of the more severe incidences of hematuria in the rats was caused by crystalline deposits in the renal pelvis.