An homologous radioimmunoassay for brown trout vitellogenin (VTG) was developed. Intact VTG, isolated from juvenile brown trout by selective precipitation and anion exchange chromatography was labelled with Na(125)I, with 1,3,4,6-tetrachloro-3α,6α-diphenylglycoluril (Iodogen) as the oxidizing agent. Incorporation of Na(125)I into VTG was higher than 75% and there was little degradation of the labelled protein. Labelled VTG eluted at the same position as unlabelled, purified brown trout VTG when analyzed by gel filtration on Sepharose 6B. Antisera with high titers, i.e. 1∶250 000, against brown trout VTG were raised in rabbits. The sensitivity of the assay was 5 ng VTG/ml and the standard curve was linear between 10 and 100 ng VTG/ml. Plasma from maturing female brown trout, as well as estradiol-treated and untreated juvenile brown trout diluted parallel to the standard curve, while plasma from maturing female rainbow trout and estradiol-treated arctic charr diluted non-parallel to the standard curve for brown trout VTG. Purified rainbow trout VTG and plasma from maturing female rainbow trout diluted parallel to each other, but with lower sensitivity than for brown trout VTG. Determinations of protein-bound phosphorus in the plasma of estradiol-treated juvenile brown trout correlated well with the RIA determinations of VTG. Repeated freezing and thawing of plasma samples yielded up to a hundred-fold increase in the apparent VTG level, while storage of a plasma sample for one year at -20°C did not affect the VTG level as measured by RIA.