The immunoreactivity of a commercial preparation of human chorionic gonadotrophin (HCG) was determined in a homologous double antibody radioimmunoassay for HCG using antisera to the beta-subunit of the hormone. The immunoreactivity of the commercial HCG was found to be 2.2 +/- 0.3 (mean +/- 2 S.D.) times the biological potency. Exclusion chromatography of the commercial HCG and then curve resolution of the elution profile derived from the radioimmunoassay revealed that on a molar basis, 21% of the immunoreactivity was attributable to beta-HCG. The rate of clearance of this preparation of HCG from the plasma after intravenous administration was determined as a function of the dose administered to ten normal men (age 36--64 years). The doses ranged from 10,000 to 300,000 i.u. immunological potency. The rate of clearance decreased significantly (r = 0.574, P less than 0.05) with increasing doses of HCG from a mean of 786 ml/h at the lowest dose to a mean of 298 ml/h at the highest dose. The renal clearance of administered HCG also decreased with increasing doses; the mean renal clearance of the 10,000 i.u. dose was 3.6 times the mean renal clearance after administration of 200,000 i.u. When the accumulated urinary HCG was expressed as a percentage of the dose administered, 14.1% of the 10,000 i.u. dose and 9.8% of the higher doses accumulated in the urine, suggesting that non-renal clearance increased with increasing dose.