Certain retinoids have reproducibly prevented carcinogen-induced cancer of the skin, mammary gland, and urinary bladder of experimental animals and, in isolated experiments, have demonstrated positive activity for carcinogen-induced cancer of the esophagus, cervix, and liver. Attempts to establish activity of retinoids in prevention of lung and colon cancer have generally been unsuccessful. In the chemotherapy of established cancers, retinoids have exhibited modest activity against those that are immunogenic. The preventive activity of retinoids appears to be associated with their role in chemotherapy is consistent with general stimulation of the immune response of the host.