SummaryThe effect has been studied of a number of seminal constituents and related substances on the motility of diluted ram, bull, rabbit, and human spermatozoa. All tests were made with a fructose phosphate diluent over a 4-hr period at room temperature.Trace concentrations « 0·20 mg/lOO ml) of copper, cobalt, manganese, iron, and zinc had little effect on ram, bull, or rabbit spermatozoa, except that copper depressed the motility of ram spermatozoa.Biotin improved the viability of bull spennatozoa but had no effect on ram or rabbit spennatozoa. Thiamine, riboflavin, niacin, inositol, p-aminobenzoic acid, pantothenic acid, and folic acid were inactive in all species.None of 21 amino acids had any significant beneficial action on ram, bull, or rabbit spennatozoa, and some were toxic.Of several miscellaneous seminal constituents, cytochrome c and spennine phosphate improved the motility of rabbit spermatozoa but had no effect on ram or bull spermatozoa. Citric acid, choline chloride, Col, and adenosine triphosphate were inactive and ascorbic acid toxic to the spermatozoa of all three species.Human spermatozoa were found to be particularly susceptible to dilution and no protective action was shown by substances that were beneficial to other mammalian spermatozoa.