A 56-year-old male presented with ascites and gynecomastia. Laparoscopy demonstrated peritoneal tumors which were biopsied. Conventional histology and electron microscopy revealed the tumor to be a malignant mesothelioma. The ascitic fluid and tumor cell lysate, but not serum, contained hCG by specific assay, and the immunoreactive hCG had characteristics similar to purified hCG in filtration on Sephadex G-100. Malignant mesothelioma is a tumor which may be associated with elevated hCG concentrations. Demonstration of hCG in ascitic fluid should suggest the presence of neoplasm.