A homologous radioimmunoassay (RIA) for Indian carp (C. catla) gonadotropin (GtH) was developed by using purified Catla GtH and its specific antiserum. In Ouchterlony agar diffusion, antisera raised against purified Catla GtH (cGtH), showed clear crossreaction. Catla-anti GtH (anti-cGtH) did not cross-reacted with Catla TSH enriched fraction. Immunocrossreaction was further confirmed with competitive binding inhibition studies where displacement of radiolabelled cGtH was precisely linear against increasing concentrations of cGtH, hence this was later used as standard curve for RIA. Competitive binding inhibition was also observed with purified murrel GtH, silver carp GtH and Cyprinus GtH, using varied doses. Both murrel and silver carp GtH showed clear parallelism with cGtH, while Cyprinus GtH inhibition slope was less steeper. Mammalian GtHs (hCG, oLH, oFSH), bTSH and bPRL had no crossreaction with anti-cGtH. Radioreceptorassay (RRA) for cGtH was developed by preparing plasma membranes from Catla oocytes. Binding of 125I-cGtH to oocyte plasma membranes showed saturability with high affinity (Ka = 0.11 X 10(13)M-1) and low capacity (17 fmol/mg protein). Displacement of 125I-cGtH in receptorassay by cold cGtH was linear and therefore served as standard curve. The interassay and intrassay variability in RIA was 7.9% and 3% while that of RRA was 5% and 3% respectively. Sensitivity of RIA was in the picogram level whereas it was in nanogram level by RRA. Determination of carp pituitary and serum GtH content by RIA and RRA showed the consistency, precision and validity of these assays. Although RRA was comparatively less sensitive than RIA, it was convenient, quick and less expensive.