Intraperitoneal chemotherapy with mitomycin C and 5-fluorouracil has been used to rescue patients from peritoneal carcinomatosis after perforated cystadenocarcinoma of the large bowel or appendix. A staging celiotomy was used to asses the results of therapy. In nine patients tumor was recovered from the abdominal cavity and was subjected to careful histopathologic examination. The posttreatment appearance of the tumor was compared with that before treatment. Also, tumor deposits harvested from these same patients, but from anatomic sites not directly exposed to chemotherapy were examined. The most striking findings observed in treated patients were as follows: (1) the marked reduction in the number of foci of atypical neoplastic epithelium lining the mucin globules, and (2) atrophy and degeneration of the atypical neoplastic epithelium. All treated patients showed these effects but they were not in all sampled tumor specimens, suggesting nonuniform drug distribution in the abdominal cavity and limited penetration of chemotherapy into tumor. The authors conclude that intraperitoneal chemotherapy produces marked histologic changes in mucinous tumors when there is direct contact between tumor and intracavitary drugs.