His first official transplantation of a monkey gland into a human took place on June 12, 1920. Thin slices (a few millimetres wide) of testicles from chimpanzees and baboons were implanted inside the patient's scrotum, the thinness of the tissue samples allowing the foreign tissue to fuse with the human tissue eventually. By 1923, 700 of the world's leading surgeons at the International Congress of Surgeons in London, England, applauded the success of Voronoff's work in the "rejuvenation" of old men.