Human antibodies against botulinum toxins A, B, and E have been raised by repeated injections of pentavalent toxoid in a healthy volunteer. The final titer was 3.2 U anti-A, 0.4 U anti-B, and 2.5 U anti-E/ml. In mice, the efficacy of the antitoxin decreases with the time between poisoning and application of the antiserum. The dose recommended for prophylactic purposes in man is 1 ml/kg. In overt poisoning, therapy should be started with higher titer animal serum since in animal experiments high titer sera can stop (although not reverse) the symptoms of overt botulism within an-admittedly not too broad-rnage of time and dosage. Later on, therapy can be continued with human antiserum. An inverse radioimmunassay for botulinum A antitoxin using labeled botulinum toxin and antibody-coated tubes is described. The serum is available upon request from the author.