ABSTRACT The assay of FSH by a radioligand receptor assay (RLA) using homogenates of rat testicular tissue incubated for 17 h at 21°C has been assessed. Chloramine-T or lactoperoxidase were used for iodination. The assay gave linear dose-response lines between 30 and 2000 ng sheep FSH/tube, and there was usually no major interference by LH. Two batches of labelled FSH, however, gave assays in which LH showed a striking interaction with FSH. When these batches were avoided and FSH and LH were mixed in ratios that differed less than fourfold, the assay was combined successfully, in the same tubes, with an RLA for LH, using LH and FSH labelled with 131I and 125I respectively. The RLA for FSH was not suitable for assay of FSH in rat serum because of apparent non-specific interference. Assay by RLA of rat FSH, in pituitary homogenates or released during incubation in vitro, gave results which were not closely correlated with those of either conventional bioassay or radioimmunoassay.