We report a patient with ovarian malignant mixed mesodermal tumor (MMMT) that produced immunoreactive human chorionic gonadotropin beta-subunit (IR-hCG beta). Qualitative analysis indicated that IR-hCG beta in the patient's serum represented the free form of the beta-subunit of hCG. Immunohistochemical localization of hCG beta in MMMT was observed in the sarcomatous stroma, but not in the adenocarcinoma component. Reverse transcription-polymerase chain reaction (RT-PCR) for hCG subunits revealed that the mRNA transcripts of two of six hCG beta genes (beta-1 and -2) were predominantly expressed in the tumor tissue. The serum level of free hCG beta correlated with disease status and was helpful for monitoring the response to therapy for MMMT in this patient.