The immunological specificity of fish gonadotropin does not generally allow the use of the homologous RIA systems prepared for salmon and carp GtH for the determination of gonadotropin in other species. Recently, an RIA using an antibody directed against the β c-GtH subunit has enabled GtH measurement in the eel. A similar approach using the s-GtH β subunit in homologous salmon systems and combinations between carp and salmon β systems has been developed. α and β subunits and the corresponding antibodies have been prepared from carp and salmon GtH. A study of the binding of the six antigens to each of the six antibodies confirms the species specificity of the α subunits. On the other hand, all the systems in which a β component is included, homologous or heterologous, for carp and salmon, are less specific and recognized c-GtH as well as s-GtH. The competition by pure antigens, partially purified pike and gilthead sea bream GtH, tilapia, milkfish and eel pituitary extracts, for the binding of s and β s-GtH, c and β c-GtH to the anti-serum directed against these four components has been studied. It was always possible to find one or several antigen-antibody combinations which were specifically competed for by each of the gonadotropic preparations. The assay has been validated for gilthead sea bream GtH by comparison of the results obtained after RIA determination and biological assay. Assays performed with perch show that it is also possible to use these systems for the GtH estimation in the plasma of this species.