In 349 patients with pure seminoma the authors determined human chorionic gonadotropin (HCG) plus the free &subunit (HCG-8) by a polyclonal radioimmunoassay (RIA) and found elevated serum levels in 42 patients (12%). In addition, each HCG and HCG--8 were specifically measured by monoclonal immunoradiometric assays (IRMA) in 19 of these patients. Overall 12 patients were HCC-@ positive and 12 HCG positive. Elevated serum levels of both molecules were found in five patients, whereas seven patients showed elevation only in HCG and another seven patients only in HCG-6. In serum samples of patients with high HCG-b and minimally elevated HCG levels the free @-subunit could be detected by gel filtration and immunoradiometric assays, whereas both HCG-8 and hob-HCG were found in ultrafiltered and concentrated urinary samples. The authors conclude that HCG-B can be present as the major form in serum even if both HCC-B and HCG are detectable in urine. When measuring hormone concentration as a tumor marker in seminoma patients, methods which determine both activities, HCG plus HCG-6, seem to be the most reliable. Whereas 16 patients with Stage I-IIc had HCG-8 levels between 0.1 and 2.8 IU/I and currently have no evidence of disease, two patients with Stage IV had HCG-b levels of 18 and 37 IU/1 and died. Current investigations have to clarify the prognostic significance of &subunit secretion.