The authors review a long experience in diphtheria prophylaxis in New York City, with the conclusion that diphtheria toxin-antitoxin mixture (T-A) has immunized 80-85% of treated children there. The newer toxoid and alum-toxoid preparations appear to be much better antigens for younger children than T-A, while the latter may be reserved for older children and adults, on account of the larger expectation of severe pseudo-reactions in these age groups. The tabulated results of animal exps. and of human immunization here given support the conclusion that alum-toxoid is the most effective diphtheria prophylactic thus far developed.