Follicular fluid samples from rabbits, cats, pigs, women and cows had acrosome reaction-inducing activity (ARIA) on rabbit spermatozoa as determined by differential staining after incubation with these fluids. Activity was retained after dialysis and at least 50% was labile when heated to 56.degree. C for as little as 20 min. Induction of the acrosome reaction by bovine follicular fluid showed a dependence on concentration of follicular fluid and spermatozoa and on Ca ions and had an approximately 8 pH optimum. Enzyme treatments showed proteases destroyed ARIA, and this activity was completely blocked by treatment of bovine follicular fluid with goat anti-bovine antiserum. EM observations indicated similarity of reactions observed to that occurring in vivo. The rabbit acrosome reaction-inducing activity of bovine follicular fluid is apparently a serum component(s), probably protein(s) of high MW.