A highly purified and potent hyaluronidase preparation extracted from bull testis was employed to induce heterologous antibody by sensitizing adult rabbits. Specific anti-enzyme antibodies were demonstrated by Ouchterlony's method, and by passive cutaneous anaphylaxis (pca), and haemagglutination and complement fixation tests. In order to localize the cellular site of the accumulation of the enzyme, direct and indirect immunofluorescent techniques were applied to tissue sections and to isolated germ cells of adult bull testis. Specific fluorescence adequately checked by control reactions appeared in the perinuclear area of spermatids, in the acrosomes of spermatozoa and with less certainty in the cytoplasm of other cells tentatively identified as spermatocytes. Non-specific fluorescence could be seen in the basal cell line of the seminiferous tubules.