Antisera made insoluble by polymerization were used for the radioimmunoassay of FSH and LH, as this was shown to be a useful method of separating bound and free labelled hormone. Highly purified HCG and FSH, prepared in this laboratory, from urinary source were labelled with 125I. Antisera were raised by injecting urinary materials. The specificity of antisera was studied and some cross-reactivity was found between anti-urinary FSH and LH, but not with HCG. The sensitivity of LH and FSH radioimmunoassay was 1 mIU/ml and 3 mIU/ml, respectively. Some determinations in dialyzed urine from various sources have also been performed and the preliminary results obtained are compared to those previously reported by others. This paper discusses the radioimmunoassay of FSH and LH using a new solid-phase method, i. e. the use of polymerized antiserum, as previously described (Donini et al. 1968).