Serum hCG levels were measured in apparently healthy nonpregnant women and men using a highly sensitive and specific time-resolved immunofluorometric assay. The sensitivity of the assay was 0.03 IU/L. The levels were low in women of fertile age (median, 0.05 IU/L) and in men less than 60 yr of age (median, 0.04 IU/L). In women the median level increased to 1.1 IU/L after the menopause (range, 0.17-4.8 IU/L), and a similar but smaller increase occurred in men after 60 yr of age (median, 0.20 IU/L; range, less than 0.03-2.3). Stimulation with GnRH caused a 2- to 3-fold increase in the hCG level in both men and women. Chronic treatment of postmenopausal women with a combination of estrogen and progestagen lowered their serum hCG levels to about 50% of the pretreatment values. The hCG in serum could be separated from LH by gel chromatography, and the hCG immunoreactivity measured by direct assay of serum corresponded to the immunoreactivity eluted in the hCG fractions after gel chromatography. Thus, the results were not due to cross-reaction with LH. We conclude that serum of nonpregnant women and men contains hCG-like material, whose production is modulated by GnRH and sex steroids.