Dr. Perez-Martin's problem regarding reactions to booster doses of tetanus toxoid is one that has concerned many physicians. Excluding those reactions due to pertussis or diphtheria antigens in combined preparations, most appear to fall into one of three patterns: apparent true allergy with urticaria, a severe local reaction (Arthus phenomenon) with or without systemic symptoms, and unpleasant, generalized fever and malaise of several days' duration. The problem has recently been superbly discussed by Edsall and others (J.A.M.A, 202:17, 1967).