Two hundred four patients with a microscopically incomplete resection for carcinoma of the colon or rectum were accepted for study. All patients were treated with 5-fluorouracil and methyl-CCNU beginning about the second postoperative week. Concurrent immunotherapy with the methanol extraction residue of bacillus Calmette-Guérin (MER) was randomly assigned to 103 patients. Treatment was continued as long as acceptable to the patient, and until clinical recurrence. Toxic reactions to the drug were not increased by the addition of MER, and seldom were severe enough to require the discrimination of therapy. No evidence of improved survival was seen in treated patients. On the contrary, survival in patients who experienced severe reactions to treatment may have been impaired by MER, with the period of impairment continuing after all adjuvant therapy was stopped. Similar proportions of treated and control deaths were attributable to residual or recurrent disease.