In 1954, less than 10 years after penicillin came into general use and had been shown to be an exceedingly effective drug against streptococcal infection in children, Maclyn McCarty1 voiced the following warning: "The extensive use of penicillin throughout childhood may ultimately result in an adult population with a susceptibility to streptococcal disease comparable to that kind we find in school children." This warning was based on the observation that treatment with penicillin interfered with the development of type-specific immunity, which is considered to be the foundation of adult immunity to streptococcal infection. Since the basic studies of Denny, Wannamaker, Brink, Rammelkamp, and Custer2 indicate that prevention of rheumatic fever can be attained by adequate therapy of the initial streptococcal infection with penicillin, this leaves the physician between the horns of a dilemma. If he treats streptococcal infections adequately he interferes with the development of type-specific immunity. If