A highly specific and sensitive radioreceptor assay for FSH has been developed, using partially purified plasma membranes from bovine testes. Highly purified hFSH was radioiodinated by the lactoperoxidase method. The limit of detection of purified hFSH was 1 ng/ml on the basis of 2× standard deviation of the zero value. The assay was applicable for measurements of FSH in serum; however, the sensitivity decreased slightly to 2.5 ng/ml. The precision of the assay was less than ± 10% within-assays and ±15% between-assays as expressed by the coefficient of variation. The assay was highly specific for FSH of various species. Slight inhibition of uptake of 125I-hFSH was observed with LH and TSH; but no competition was seen with 10,000 ng/ml of insulin, prolactin, hGH, hCG, and subunits of LH. Purified hFSH (LER-1575C) was measured to be 200× the potency of the reference FSH/LH (LER-907); and purfied ovine FSH (LER-1491) and rat FSH (FSH-I-1) were estimated to be 35× and 71× that of oFSH-S-1 (NIH), respectively. The content of FSH in human pituitary was estimated to be 226.8 ± 118.8 mIU/mg, and the index of discrimination (radioimmunoassay/radioreceptor assay) for pituitary FSH was demonstrated to be 1.71 ± 0.49. For measurements of serum hFSH, the index of discrimination (radioimmunoassay/radioreceptor assay) was demonstrated to be 1.08;± 0.20.