By radiolabeling using NaIO4/[3H]KBH4, a new sialoglycoprotein with Mr of 19,000 was found in the secretion of rat seminal vesicle. It was shown to interact non-covalently with semen coagulum. It existed in three acidic forms with pI values of 4.1, 3.7 and 2.9 and possessed high contents of sialic acids and acidic amino acids.