The most undesirable side effect of a new gestagen intended for the treatment of pregnancy disturbances is the virilization of female foetuses. In 1961, our drug company developed a new highly effective gestagen, cyproterone acetate (figure 1) (Wiechert & Neumann 1965). We were given the task to test this substance in animal experiments for desirable and undesirable effects, particularly for the occurrence of virilization. As this drug was intended for the treatment of pregnancy disturbances, such as threatening or habitual abortion, it had to be free of virilizing side effects.