We have used high-specificity and precision immunofluorometric assays to measure the elimination half-times of human chorionic gonadotropin (hCG), hCG alpha, and hCG beta in serum over 21 days after delivery in six women with term pregnancies. Baseline concentrations and half-times were calculated with the use of a curve-fitting algorithm for multiexponential decay. In contrast to the two-component model, a three-component exponential function with baseline provided a fit for which predicted values could not be distinguished from the observed values by analysis of variance. Median half-times were 3.6, 18.0, and 53.0 h for hCG; 1.0, 23.4, and 194 h for hCG beta; and 0.6, 6.2, and 21.9 h for hCG alpha. The mean ratio of hCG alpha to hCG decreased rapidly from 36.9% to 3.3% on day 3; thereafter it increased to 64.3% 21 days after delivery because of a higher baseline concentration of hCG alpha. hCG beta had the slowest total elimination rate, and the ratio of hCB beta to hCG in serum increased from 0.8% before delivery to 26.7% after 21 days. If the metabolism of hCG and hCG beta is similar in patients with trophoblastic disease, the ratio of hCG beta to hCG must be evaluated with caution in samples taken several days after initiating therapy. We conclude that the disappearance of hCG beta from plasma is slower than previously recognized and that the ratios of hCG beta or hCG alpha to intact hCG vary as a function of postpartum time. Such information may be important in clinical studies of pregnancy disorders.