The level of human chorionic gonadotropin (hCG)-like substance in normal serum was measured using 50 μl of serum specimen by a highly specific and sensitive sandwich enzyme immunoassay for hCG which did not significantly cross-react with hLH. The level was calculated using a standard hCG and expressed in mIU/ml as that of hCG. In normal male (20–59 years of age) and normal, non-pregnant female (20–42 years of age) adults, the level was below 0.6 mIU/ml with mean levels of 0.16 mIU/ml (n=209) and 0.14 mIU/ml (n=36), respectively. In older subjects, however, the level tended to rise. In normal male adults, aged 60–91 years, the level was mostly below 0.8 mIU/ml (mean=0.56 mIU/ml, n=99), but levels of 15 specimens out of 99 were 1.1–5.5 mIU/ml. In normal, non-pregnant female adults, aged 45–88 years, the level was widely distributed between 0.1 and 11.7 mIU/ml (mean=1.6 mIU/ml, n=141). In these older female adults, levels of hCG-like substance were not well correlated to either levels of hLH and hFSH, while levels of hLH and hFSH were well correlated to each other.