Pentanal methylformylhydrazone (PMFH) given in propylene glycol as 52 weekly intragastric instillations on a 50 micrograms/g body weight basis, induced tumors of the lungs, liver and preputial glands in Swiss mice. The tumor incidences in these tissues were 72, 16 and 0% in the treated females, while these tumor incidences in the males of this group were 60, 2 and 12%, respectively. The corresponding tumor incidences in the propylene glycol instilled solvent control females were 26, 0 and 0%, whereas in the males of this group they were 22, 0 and 0%, respectively. Histopathologically, the tumors were classified as adenomas and adenocarcinomas of the lungs, benign hepatomas and squamous cell papillomas and carcinomas of the preputial glands. Pentanal methylformylhydrazone is an ingredient of the edible wild mushroom Gyromitra esculenta which is consumed by a segment of the human population around the world.