Proteins from the secretion of rat seminal vesicles were fractionated to yield (i) insoluble protein, (ii) basic proteins (soluble in the presence of urea) and (iii) neutral plus acidic proteins. The first two are necessary for the formation in vitro of a coagulum which is very similar to the copulatory plug. Collagenase-like peptidase from rat testis degrades both protein fractions and prevents coagulum formation.