Eleven normally menstruating women, who had earlier been immunized with NII beta-hCG vaccine but had no detectable anti-hCG antibody titres, were selected as controls for the hCG challenge test using 1000/2000 I.U. The test was repeated in five of them after a booster immunization, which raised antibody titres to 18-450 ng/ml. Stimulation of serum progesterone secretion was used as an index of corpus luteum (CL) response to the I.V. hCG. In the control group, the progesterone (P) secretory response following hCG stimulus showed peak levels which were significantly higher than basal levels in all except 2 subjects. The non-responsiveness in 2 subjects cannot be easily explained but may be dose-related. No significant difference was noted between the two dose levels. Length of luteal phase was increased by 4-5 days in 6 out of 20 cycles studied. The results of this test in 5 women before and after the vaccine boosters were encouraging as peak P levels appeared higher than basal levels in controls, but not so in the immunized group. However, these results could not be confirmed statistically. Nonetheless, this study is suggestive that the antibodies generated by this vaccine were capable of intercepting the effect of exogenous hCG in the human female. Further studies with more subjects and higher dosage of hCG are called for.