Alfred Jost (1916–1991) was a French endocrinologist, famous for his discovery of the Mullerian inhibitor, now called anti-Mullerian hormone (AMH) or Mullerian inhibiting substance (MIS). Alfred Jost resolved the controversy surrounding the mechanism of somatic sex differentiation by proving that male characteristics must be imposed on the fetus by the testicular hormones testosterone and AMH, respectively responsible for the virilization of the Wolffian ducts, urogenital sinus and external genitalia and for the regression of Mullerian ducts. In the absence or inactivity of these hormones, the fetus becomes phenotypically female. Alfred Jost was also a pioneer through his work on testicular differentiation, in collaboration with Solange Magre. He was the first to show that testicular organization is heralded by the development of pre-Sertoli cells, which progressively surround germ cells to form seminiferous tubules. Alfred Jost did not rely only on his brilliant mind. He distrusted theories built on suboptimal experimental data and believed that intelligence was powerless without technical skill. His search for elegant, effective techniques led him to apply surgical methods to fetal endocrinology. He was also a fascinating teacher, particularly for pre-doctoral students. He died aged 75, having retired from the Collège de France, but still active as the Secrétaire Perpétuel of the French Science Academy.