Basically, the genetic markers of semen are partitioned into two categories: the cell surface antigens and the proteins and enzymes. These categories reflect the chemical nature of the markers and the methods used for their detection. The cell surface antigens, for example, are detected by immunological procedures, usually agglutination reactions or some variation thereof. In the context of the discussion, the soluble ABO antigens found on the soluble secretor substances are considered 'honorary' cell surface antigens. Genetic variation of proteins and enzymes, on the other hand, is usually detected by electrophoretic procedures. Both intracellular and extracellular proteins and enzymes are included in this category. Also included are the immunoglobulin markers Gm and Inv, for although they are detected by immunological procedures, they are carried on soluble proteins. Neither category of genetic variation is without unresolved problems. The genetic typing of semen is not nearly as straightforward as the genetic typing of blood. There are unique problems of sample collection, sample dilution, contamination, and degradation. Recognition of these problems is the first step toward their solution, and with continued research, progress toward the practical use of genetic typing of semen will be made. Photographic illustrations and 38 references are provided. (Author summary modified)