Two transglutaminase-mediated modifications of the rat epididymal spermatozoon surface were demonstrated in vitro. Transglutaminase was effective in promoting the binding of spermidine to the sperm. Moreover, the enzyme, by reacting with one of the major proteins secreted by the rat seminal vesicle epithelium, produced a modified form of the protein with a higher molecular weight and the capability of binding to the sperm cells. A specific physiological role for the enzyme, bringing about modifications of the rat sperm surface in the seminal fluid environment, is suggested.