After repeated im courses of menopausal gonadotropins and hCG, a factor was found in a woman's serum which preferentially bound the intact hCG molecule (nanograms or milliinternational units required for 50% displacement of [125I] hCG: hCG, 9.2 ng; APL, 74 mIU; Pergonal, 400 mIU; LER 960, 900 ng; and FSH and TSH alpha-subunits, hCG beta-subunit, and LH beta-subunit, greater than 1000 ng). This factor had a low affinity (Ka = 5.8 X 10(9) liters/M) and high capacity (binding capacity, 5.5 X 10(-11) M/liter) for hCG, was isolated with the immunoglobulin G fraction of the patient's serum, and coeluted with immunoglobulin G from a Sepharose CL-6B-200 column. Preincubation of hCG with this fraction did not reduce the in vitro biological activity of the hCG in the hypophysectomized rat ventral prostate weight assay. Similarly, there was no gonadal resistance to hCG in the patient, since ovulation could still be induced with hCG, and both progesterone and 17-hydroxyprogesterone levels as well as duration of the luteal phase increased after repeated hCG injections. This serum factor prolonged the half-life of injected hCG in the patient's circulation (t 1/2 = 9.4 days in patient vs. 1.3 days in controls). The titer of the factor transiently decreased with the exogenous administration of hCG. No binding of the serum factor to human placental tissue could be demonstrated by immunohistochemical techniques. Thus, this factor behaves similarly to the antibodies frequently found after the injection of other polypeptide hormones, in that it serves as a high capacity reservoir of the hormone but does not significantly reduce its biological activity. These results also indicate that the factor requires the intact tertiary structure of the hCG molecule for immune recognition.