[en] A double isotope modified Farr assay was used to determine the total binding sites and affinity of antibodies to human chlorine gonadotrophin. Precipitation of the antigen-antibody complex at equilibrium with ammonium sulphate gave very high levels of nonspecific binding. Good discrimination over background was observed using a specific anti-immunoglobulin serum. However the interest was in measuring the affinity of antibodies raised in several animal species so it was more appropriate to use a single nonspecies specific precipitating reagent. It was found that the use of a mixture of acetate gave very low levels of non-specific ethanol-ammonium binding in baboons, marmosets, rabbits and mice. (Auth.)