In 1903 Ancel and Bouin1 published the theory that the interstitial cells of the testis (the cells of Leydig) produced the testicular hormone. The theory was based upon 4 main premises, as follows: Animals remain sexually active (a) after vasectomy, (b) when they possess only cryptorchid testes, (c) when their testes have been exposed to Roentgen rays and (d) when the only testicular tissue they possess is a graft. Under each of these conditions all germinal epithelium was reported absent while the interstitial cells were normal or hypertrophied. All 4 of these premises have been shown to be incorrect (see Oslund4, 5, 6, 7) and some germinal epithelium usually exists under each of the above conditions. Since both germinal epithelium and interstitial cells are present in hormone producing testes it has seemed impossible to designate which of these produces the hormone.It occurred to the writer that if a potent extract could be obtained from the epididymis this would offer strong presumptive evidence that t...