A new compound, APAZA, consisting of a molecule of 5-aminosalicylic acid linked to one molecule of 4-aminophenylacetic acid by an azo bond, was testedfor its ability to inhibit acute colitis in rats caused by Clostridium difficile toxin A. When administered chronically for 5 days in drinking water, APAZA significantly inhibited toxin A-induced myeloperoxidase activity, luminal fluid accumulation, and structural damage to the colon at doses of from 1 to 100 mg/kg x day. For comparison, sulfasalazine was administered in identical doses and was found to significantly inhibit toxin A-induced colitis only at the dose of 100 mg/kg x day. When 4-aminophenylacetic acid alone was administered chronically in drinking water, it also inhibited toxin A-induced colonic inflammation at a dose of 100 mg/kg x day. In order to determine if 4-aminophenylacetic acid has a direct anti-inflammatory effect on the colon rather than a systemic effect, 4-aminophenylacetic acid was administered acutely to surgically prepared isolated colonic segments by intraluminal injection in anesthetized rats 30 min before toxin A was injected. 4-Aminophenylacetic acid strongly and significantly inhibited toxin A-induced colitis in this experiment at doses as low as 10 microg/segment. It is concluded that APAZA is a potent inhibitor of toxin A-induced colonic inflammation in rats and that its constituent, 4-aminophenylacetic acid, is responsible for this increased protection against colitis compared to the 5-aminosalicylic acid component of sulfasalazine.