The present invention relates to a method of treating infertility in a female mammal that involves controlled ovarian hyperstimulation. The method includes administering to the female a combination of (i) an FSH substance in an amount effective to stimulate follicular development and (ii) a steroid in an effective amount to inhibit or suppress the secretion of luteinising hormone. The steroid can be substances represented by the following formula in which formula R1, R2, R3, R4 independently are a hydrogen atom, a hydroxyl group or an alkoxy group with 15 carbon atoms; each of R5, R6, R7 is a hydroxyl group; no more than 3 of R1, R2, R3, R4 are hydrogen atoms; derivatives of the aforementioned steroid substances; or mixtures of one or more of the aforementioned substances or derivatives.