PURPOSE:The protective effect of 5‐aminosalicylic acid against colon carcinogenesis was investigated.METHODS:Eighty female F344 rats aged seven weeks received an intrarectal dose of 2 mgN‐methylnitrosourea dissolved in 0.5 ml of water three times weekly for five weeks to induce colon cancer. Beginning at 6 weeks after the last dose ofN‐methylnitrosourea, the rats were treated with an intrarectal dose of 1 mg 5‐aminosalicylic acid suspended in 0.5 ml of vehicle solution (0.3 percent water solution of methylcellulose) three times weekly for 15 weeks.RESULTS:Colon cancer incidence and mean number of tumors per rat at the end of the 15‐week treatment period were significantly lower and smaller in the 5‐aminosalicylic acid‐treated group (10 percent and 0.2) than in the vehicle‐treated (80 percent and 1.6) and untreated (68 percent and 1.1) control groups. However, the mean numbers of tumors per tumor‐bearing rat were comparable: 1.5, 2, and 1.6. No distinct differences among the groups were observed in the tumor pathology with respect to their location (within 0‐10 cm proximal to the anus), shape (plaque shaped or polypoid), size (<10 mm in diameter), invasion (restricted to the mucosa or submucosa), or histologic type (differentiated adenocarcinoma).CONCLUSION:Our results indicate that 5‐aminosalicylic acid administered directly into the colonic lumen strongly suppresses the promotion stage of colon carcinogenesis.