To the Editor.— I would like to respond to the question regarding the Arthus phenomenon following immunization with diphtheria-pertussin-tetanus (DPT) in the QUESTIONS AND ANSWERS section (238:2307, 1977). As noted by Zack Haddad, MD, the incidence of the Arthus-type response diminished when the interval between tetanus toxoid boosters increased. I think there should be elaboration on this. According to Goldsmith et al,1the minimum protective level of serum antitoxin is 0.01 units/ml. This is readily achieved by recommended immunization procedures, and, in fact, an appropriate recall response is made by adequately immunized patients in 24 hours or less following a tetanus toxoid booster given even several years after the previous booster.2 Edsall et al (202:111-113, 1967), however, have shown that the antibody levels in many children referred for routine or emergency tetanus toxoid booster already were 40 to 2,500 times the minimum protective level. The incidence of reactions,