Bull-testis hyaluronidase acting to disperse the follicle cells of rat ova in vitro was inhibited by blood sera of the following animals, roughly in a decreasing order of effectiveness: (1) rat, chicken (2) rabbit, horse (3) Limulus, man (4) cow. When rat-testis hyaluronidase was used, only a small uniform inhibitory action by these sera was observed. The failure of rat serum to prevent conception when introduced into the rat uterus was explained by these results. The inhibitory activity of rat serum on bull-testis hyaluronidase can be overcome by increasing the amt. of the enzyme. Rat serum, containing antibodies against bull-testis enzyme, did not inhibit rat-testis enzyme to a degree greater than that obtained with normal rat serum. Women, diagnosed as sterile, had normal amts. of inhibitor in their blood serum. A method for standardization of hyaluronidase by the rat-ova test is presented. A rat-ova unit of hyaluronidase is defined as the amt. of enzyme in .2 ml. of Ringer''s soln. which will disperse the follicle cells of 2 or 3 or 50% of a greater number of ova (not exceeding 8) within 3/4-1 hr. The follicle cells are considered dispersed when not more than a single layer of cells remain surrounding an ovum.