The objective of this paper is to present a new approach to estimation of the hazard of toxic chemicals to the human male reproductive capacity, based on data that can be obtained from experiments on animals. Mechanisms by which an agent causes infertility will be considered and endpoints that can be quantitatively related to human infertility will be used. The approach is as follows. First, a dose-effect relationship for the agent and endpoint chosen in the experimental animal should be determined. Next, a corresponding dose-effect relationship on the same endpoint in man can be calculated using an extrapolation factor, which relates that dose given to a test animal to the dose required to produce an equal effect in man. Finally, the alteration of the endpoint in man is used to calculate the increase in the incidence of infertility in the population.