Male Wistar rats were given repeated injections of sterigmatocystin (stg) and related compounds into the peritoneal cavity once a week for 23 weeks. Only stg alone produced mesothelioma in 20 of 40 rats given a total dose of 20 approximately 25 mg, while no mesotheliomas were found in rats injected O-acetyl-stg (AcO-stg) and dihydro-stg. There were two histological types in the mesothelioma induced by stg; the epithelioid and mesenchymal types. There was little loose connective tissue between the epithelioid cell tumors, whereas abundant dense collagen fibers were found between the cell nests of the mesenchymal cell types of mesothelioma. The hepatocellular carcinomas were found in a rat treated with stg and in five animals given AcO-stg. The fact that mesotheliomas could be induced in high incidence by direct intraperitoneal application of stg suggests a potent carcinogenicity of stg. Although both dihydro-stg and stg formed fine needle-like crystals when injected into the peritoneal cavity, dihydro-stg could not produce any mesothelioma. Dihydro-stg, reduction product of stg, has no double bond in the terminal furan ring. Therefore, the carcinogenicity of stg may be related more closely with the presence of a double bond in the terminal furan ring of stg than the physical form in the peritoneal cavity. AcO-stg may easily be absorbed from the peritoneum, so that it could not persist in the peritoneium for sufficient duration for carcinogenesis.