The prevalence of human chorionic gonadotropin (hCG) was estimated by measuring immunoreactive hCG in plasmas and tumor tissues from patients with various neoplasms. To detect small amount of plasma hCG in the presence of luteinizing hormone (LH), plasma hCG level was measured by heterologous radioimmunoassay using anti-hCG-beta antiserum and compared with plasma LH level measured by heterologous radioimmunoassay using anti-LH-beta antiserum. All 56 samples obtained from control subjects were found to be negative for hCG, while 10 of 100 plasma samples from patients with malignancies were positive for hCG. The prevalence of hCG in 64 tumor tissues was 42% (27/64); it was 32% (8/25) in so-called amine precursor uptake and decarboxylation (APUD) tumors and 49% (19/39) in non-APUD tumors. The difference in the prevalence of hCG in APUD vs. non-APUD tumors was not statistically significant. However, the amounts of hCG in APUD tumors were found to be less than 50 ng/g wet tissue, whereas those of non-APUD tumors ranged from several ng to thousands of ng/g wet tissue. These results suggest the APUD tumors produce less amounts of hCG than do non-APUD tumors.