VPS, a hot water extract of the Coriolus versicolor mushroom, was given at a 2% dose level in the diet of female Swiss Webster CFW outbred mice in a serial sacrifice experiment. The mice were also administered either 1,2-dimethylhydrazine dihydrochloride (1,2-DMH) as ten weekly subcutaneous (s.c) injections of 20 microg/g body weight or physiological saline (PS) as ten weekly (s.c) injections of 0.01 ml/g body weight. The animals were sacrificed at 26 weeks or 35 weeks after the first injection of 1,2-DMH or PS. The number of mice with large intestinal tumors and the total number of these tumors were: Group I (1,2-DMH), 29 and 438; Group 2 (VPS + 1,2-DMH), 29 and 344; Group 3 (VPS + PS), 0 and 0; and Group 4 (PS), I and 1, in the mice sacrificed at 26 weeks. The corresponding tumor incidences in mice sacrificed at 35 weeks were: Group 1 (1,2-DMH), 30 and 323; Group 2 (VPS + 1,2-DMH), 29 and 521; Group 3 (VPS + PS), 1 and 2; and Group 4 (PS), 0 and 0. Histopathologically, the tumors were diagnosed as polypoid adenomas and adenocarcinomas of the cecum, colon and rectum. Contrary to expectations, the VPS treatment enhanced the development of large intestinal tumors induced by 1,2-DMH in animals sacrificed at 35 weeks after the first injection of the carcinogen.