Female marmoset monkeys that are actively immunised against hCG beta-subunit remain infertile while antibody titres are high. With declining antibody levels the animals experience recurrent abortions that occur progressively later as the levels continue to wane. After booster immunisations the animals become infertile once more. The affinity and total binding sites of antibodies to beta-hCG were monitored after booster injections in marmosets and the values were correlated with subsequent reproductive events. The relationship between antibody amount and affinity varied considerably and the affinity was the important factor in producing the biological effects, pregnancies being often associated with a fall in antibody affinity. The antisera were also biologically active in inhibiting hCG-induced ovulation and increase in uterine weight in mice. There was no apparent cross-reaction between the antisera and human luteinizing hormone as tested by indirect immunofluorescence on adult human pituitary sections.