Even crystalline preparations of follicular hormone from mare''s urine contain an unknown precursor which can be converted by hydrogenation to a product which causes marked growth of the seminal vesicles. Preparations with 1 mouse unit in 2O[gamma] but inactive in the comb test can be obtained, an activity 5 times that of testosterone. From wool fat and from extracts of cinchona bark fractions can be obtained with 1 mouse unit in 100-20O[gamma] without hydrogenation, and with no action in the comb test. While the previously described [male] hormones show a ratio of 1 capon unit to 100 mouse units, these new preparations have a ratio of 10:1 and therefore belong to a new group of hormones.