Abstract
A factor, probably a protein, which depresses viability of sperm has been demonstrated in bovine seminal plasma. Its action appears to be directly on sperm and is not due to degradation products of components of the diluent. Greatest toxicity was found to be associated with that part of seminal plasma precipitated by 35% acetone. There is some evidence that the toxic principle is absent from epididymal plasma. It is heat-labile, being completely inactivated by heating to 90 C for 1 min. There is evidence that the toxic effect may be due to its sulfhydryl binding capacity.