CAPACITATION, or the ability for immediate penetration of the ovum, occurs when epididymal sperm of the golden hamster (Mesocricetus auratus) are incubated in the presence of follicular fluid1–3. Yanagimachi has reported that sperm motility is maintained by a dialysate of detoxified bovine follicular fluid, but capacitation of the sperm does not occur unless a water-soluble non-dialysable fraction is also provided4. The chemical nature of these factors has not yet been determined. Because ovarian follicles, particularly those in atresia, are known to contain lysosomal enzymes5, we investigated the effect of a lysosomal preparation from beef liver on hamster epididymal sperm. Finding a striking stimulation of motility and capacitation of the sperm, we went on to test the lysosomal enzyme, β-glucuronidase, which is widely distributed in mammalian tissue including cells of the ovarian follicle6. We found that this enzyme duplicated the effect of the lysosomal preparation, as we shall now describe.