Guanidinobenzoatase (GB), a serine proteinase with a molecular weight of 71,000, is found both free in the epididymal fluids of the mouse and bound to the sperm surface. Microgram quantities of the enzyme, purified from epididymal fluid, will completely disperse follicle cells from freshly ovulated oocytes after 15 min of incubation. Purified GB exhibits no hyaluronidase activity as determined by the acid albumin assay. The ability of GB to disperse follicle cells is blocked by a proteinase inhibitor endogenous to the male reproductive tract. The inhibitor has no effect on bovine testicular hyaluronidase. Although the function of GB has not been defined, the observations presented here indicate that it may play a role in cumulus matrix penetration during fertilization.