Comparisons were made between various LH and FSH radioimmunoassays currently being used to measure avian hormones. The two LH assays were the homologous chicken system of Follett et al. (1972) and the turkey assay of Burke et al. (1979) . These assays were also used in heterologous arrangement by interchanging the iodinated LH fractions and antisera. Five FSH assays were analyzed: two homologous chicken systems ( Scanes et al., 1977 ; Sakai and Ishii, 1980), an assay based on mammalian materials (rat FSH and anti-ovine FSH; Follett, 1976), a heterologous system using labelled chicken FSH with the anti-ovine FSH antiserum, and one using labelled turkey FSH (Burke et al., 1979) with an anti-chicken FSH antiserum. The potencies of purified chicken and turkey gonadotrophin preparations and of a range of plasma samples from Japanese quail were measured in each assay. The two LH systems showed some degree of species specificity, such that chicken LH was more active than turkey LH in the chicken assay, whereas the reverse was true in the turkey LH assay. The potency estimates of the purified hormones in the various FSH assays were very consistent. The qualitative changes in plasma hormone levels were similar in all assays, although there were some differences in the magnitude of the responses.