By intradermal testing in normal and immunized animals it has been shown that both the thermolabile and the thermostable toxins of H. pertussis are definitely although weakly antigenic in rabbits but apparently not in human beings. In normal animals, and in a few humans tested, the agglutinogen shows no primary toxicity but in immunized rabbits and human beings an immune response lasting a day or two is obtained. The toxins, on the other hand, produce a primary toxic reaction in normal animals which lasts for one week and longer; the thermolabile toxin (TLT) produces necrosis. In immunized animals the prolonged primary toxic reaction to the toxins is not shown; the reaction appears as a non-necrotic lesion of duration corresponding to the immune response produced by the agglutinogen. The above-mentioned effects were produced in animals immunized slowly and in a prolonged series of injections either by the subcutaneous or intravenous route with either live or formalin-killed whole organisms or sonic extracts. The presence of antitoxin in the sera of the immunized animals was determined by neutralization tests in the skin of normal rabbits. By this procedure no antitoxin could be detected in human hyperimmune or in convalescent sera containing agglutinins. Both toxins were obtained from both phase III and IV and para-pertussis organisms, but in much smaller amounts than from phase I strains. By neutralization tests the toxins from the several phases and in para-pertussis were shown to be related in antigenic specificity. The fallacy of using toxicity as a sole criterion in evaluation of strains suitable for the preparation of vaccines is discussed. The possible use of the various components of H. pertussis as reagents in dermal tests for susceptibility, in diagnosis and as immunizing agents is discussed.