THERE is a demand for an easy and accurate method of assaying the luteinizing fraction of gonadotrophic hormone. The present method depends upon the formation of corpora haemorrhagica in the ovaries of immature female mice after rendering their Graafian follicles mature. The gonadotrophic materials used were pregnant mare serum (‘Antex’, Leo) as a source of follicle stimulating hormone, and chorionic gonadotrophin (‘Physex’, Leo) as a source of luteinizing hormone. The material used for assay was dissolved immediately before use in sterile saline and administered as a single dose to avoid deterioration of the hormone by storage even when refrigerated1.