The biochemical analysis of human semen is based on assays of certain compounds in the ejaculate, which are secreted by the prostate (acid phosphatase, citrate, zinc), the seminal vesicles (fructose), and the epididymis (free carnitine). The information provided by seminal biochemistry is relevant not only for the investigation of male sterility, but also to endocrinology, urology and pharmacology. In the case of excretory azoospermia, the carnitine level is lower than 300 nmoles, which allows a differentiation between secretory and excretory azoospermia. Occlusions can be located by measuring prostatic and vesicular markers: they are normal if the occlusion is deferential or epididymal; if the fructose level is zero and the level of prostatic markers is high, there is an occlusion in the ejaculatory ducts or a bilateral deferential agenesia. The information provided by seminal biochemistry in the investigation of male sterility remains limited. It will probably be enhanced when spermatozoon biochemistry is better understood.