Semen quality is said to have been declining over the past 50 years. The biological significance of these changes is emphasized by a concomitant increase in the incidence of genitourinary abnormalities such as testicular cancer and cryptorchidism. The increase in regional frequency of testicular abnormalities over a relatively short period of time may be due to local environmental factors, including the comparatively recent fashion for wearing tight-fitting underwear. Data also indicates that prenatal exposure to environmental agents can affect the development of the male genital tract. From the reproductive point of view, an increased environmental impact on the human male gonad is of concern and merits the development of sensitive techniques that can detect deleterious agents.