Semen represents the suspension of spermatozoa in the seminal plasma. Spermatozoa are produced by the testicles, and the seminal plasma by the secretion of the accessory glands. The amount of ejaculate, the concentration of spermatozoa, and the chemical composition of seminal plasma varies among species, depending mostly upon the anatomy and physiology of the accessory glands. Extensive biochemical studies have broadened our knowledge of the great variety of chemical constituents of semen. The pioneer work of Mischer on salmon spermatozoa opened the door for the investigation of nucleic acid chemistry and consequently of the chemistry of hereditary material. Enzymologists established the existence of a number of enzymes in seminal plasma, the role of some of which is known and of others is dubious. Fructose, instead of a more generally occurring glucose, was found to be the energy supplying carbohydrate. The characterization of the proteins of seminal plasma has not been fully investigated in species other than man. Only one worker has investigated the proteins of bovine seminal plasma. Introduction of artificial insemination as the preferential method of insemination in cattle emphasized the importance of selecting bull donors with regard to both their genetic quality and their fertilizing capacity. Also, the long-time hope of sex control became revived in respect to the potential possibility of treating semen prior to its use and consequently increasing the incidence of the desired sex. Many methods have been tried, of which the moving-boundary electrophoresis appeared to be the moat promising. Recent developments in electrophoretic methods resulted in the introduction of electrophoresis in stabilised media as superior to the moving-boundary type for the separation of protein mixtures. The two most frequently used media in this type of electrophoresis are filter paper and starch gel. Both the proteins of seminal plasma and spermatozoa lend themselves to electrophoretic studies by either electrophoresis in stabilized media or moving-boundary electrophoresis. For this reason the studies presented here have been divided into (l) the characterization of proteins of seminal plasma by means of filterpaper and starch-gel electrophoresis and (2) the electrophoretic study of spermatozoa by moving-boundary electrophoresis. The latter study was primarily concerned with the development of the type of electrophoretic cell suitable for electrophoresis of spermatozoa and the selection of a buffer which would have no adverse effect on the fertilizing capacity of spermatozoa. Only preliminary observations on the sex ratio of the offspring of cattle that were inseminated with the Electrophoretically treated semen are reported here because this aspect of the problem constitutes another and a long term project by itself.