A quantitative assay for human chorionic gonadotrophin (HCG) based on the competition between isotopically labelled HCG and unlabelled HCG for HCG antibody is described. Antibody-bound HCG is separated from free HCG by precipitation with anti-\g=g\-globulin serum. As a result of an immunological cross-reaction the assay also measures pituitary luteinizing hormone. It detects 0\m=.\001 i.u. HCG/ml. and normal urinary luteinizing hormone concentrations can be estimated without extraction. The specificity of the assay is discussed.

Radioimmunological cross-reactions between human chorionic gonadotrophin and heterologous plasma and pituitary extracts from various species have been examined. Where serial dilutions of a sample do not yield a curve parallel with similar dilutions of the reference standard, quantitative estimations are not possible, but relative potencies can be determined. Such relative potencies against purified heterologous gonadotrophins may be useful for the study of species whose gonadotrophins have not been purified adequately for isotopic labelling in a species-specific radioimmunoassay.