EXPERIMENTS on the antigenic properties of iodinated proteins prompted the question of whether tadpoles could be passively immunized by adding antiserum to the surrounding medium. While investigating this problem, certain experiments were carried out with anti-tadpole serum prepared by injecting tadpole extracts into rabbits and tested by adding to water containing tadpoles. The results were reminiscent of those obtained by Oliver-Gonzalez1 with antiserum to parasitic worms.