A specific and sensitive homologous radioimmunoassay (RIA) was developed for the measurement of toad (Bufo japonicus) prolactin (PRL). PRL isolated from toad pituitary glands was used for generating antiserum in a rabbit, for radioiodination, and for the standard. Several dilutions of plasma and a homogenate of the anterior lobe of the pituitary gland of toads yielded dose-response curves which paralleled the standard. Plasma from hypophysectomized toads showed the least amount of cross-reaction. Bovine PRL, ovine PRL, ovine growth hormone, and a homogenate of toad neurointermediate lobes showed no inhibition of binding even at relatively high doses in this RIA. A preliminary application of homologous RIA for toad PRL was performed by determining plasma and pituitary PRL levels in adult toads of both sexes captured in spring and late summer. Plasma PRL levels of male toads remaining in the pond for breeding were significantly higher than those of the males feeding in the bush or hibernating under the ground. The pituitary PRL content in spring toads of both sexes was higher than those in summer toads. The pituitary PRL content in females was invariably lower than those in simultaneously captured males.
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