The follicle-stimulating hormone (FSH) content of urinary gonadotrophic extracts was assayed by its effect on the ovarian weight of immature mice when given in conjunction with 40 i.u. human chorionic gonadotrophin. About three-quarters of all routine assays gave values of λ between 0·15 and 0·30. Precision was slightly increased when the material was given in three rather than in five injections. Correction of ovarian weight for body weight was either invalid or of no value in reducing variance. Removal of between-litter variance increased precision considerably. Mice of three randomly bred colonies were all satisfactory, and inbred C57BL mice were also suitable for the assay. C3H mice were less sensitive.
The efficiency of different methods of extracting FSH from urine was examined. The method of Johnsen (1958) using precipitation with tannic acid was considered the most satisfactory and gave extracts of high potency and low bulk.
Limited experiments in which purified human pituitary FSH was assayed with and without added luteinizing hormone, gave results compatible with the assumption that the method is specific for FSH.