Six women, electively sterilized by tubal ligation and having regular ovulatory menstrual cycles, were immunized with human chorionic gonadotrophin (HCG) hapten-coupled with the diazonium salt of sulphanilic-acid. Ten mg of altered homone were injected at 2-week intervals, twice subcutaneously in oil, then twice intradermally in saline. Serum follicle stimulating hormone (FSH), luteinizing hormone (LH), estrone (E₁), estradiol (E₂) and progesterone (P₄) levels were determined by radioimmunoassay before and after immunization. After treatment, antibodies to unaltered HCG were produced in all subjects. Serum levels of LH were reduced and alterations in midcycle LH peaks were observed in all subjects. Patterns of serum progesterone were suggestive of ovulation in 2 subjects following treatment. Serum E₁ and E₂ patterns were variable. Four women were studied 6 months after the initial injection and 3 showed ovulatory P₄ patterns despite the absence of a midcycle serum LH rise in 2 subjects. The presence of simultaneous detectable LH and anti-HCG antibodies in the radioassay system is unexplained. Why apparent ovulation occurred in the absence of an immunologically detectable LH peak at mid-cycle is speculative. Practical use of active isoimmunization may prove of considerable value in the study of reproductive mechanisms and perhaps in fertility regulation in humans.