In view of the well known sensitivity of the seminiferous epithelium to various stress factors and to chemicals one would expect that semen analyses would be part of many screening programs to detect dangerous chemicals and environmental hazards. This is not the case, and our knowledge about the production and functional properties of human spermatozoa is mainly based on analyses of specimens from men with barren marriages. A more common use of semen analyses should enable us to better define the normal limits for many of the potentially relevant variables. Due consideration must, however, also be given the time for spermatogenesis and for transit through the epididymis as well as the influence of seminal plasma factors on many functional properties of the spermatozoa. From the scanty information available one can already now presume that careful analyses of motility and morphology of the spermatozoa under standardized conditions will be of help in the early detection of environmental hazards. A more common use of methods for the assessment of such functional properties of the spermatozoa as structural stability, membrane permeability, metabolism, and resistance to physical stress will give additional information about the effects of chemicals and other factors. To exploit these potential methods for the early detection of environmental hazards there is, however, also a need of a changed attitude towards semen analysis from the medical profession as well as from the public.