Understanding of streptococcal infections in man has been hindered by the existence of a wide variety of clinical manifestations, which stem from the complicated nature of the infectious agent, the difference in the reactivity of man according to age or in the past experience of infections, the characteristic infectious process, etc. Sero-epidemiological study will yield worthwhile informations which have not been disclosed by usual bacteriological or clinical investigations. This way of approach is usually accompanied with characteristics and limitations depending upon the sort of antibody to be chosen.
Among various antibodies produced during human streptococcal infections, antistreptolysin O (ASL) has been known to be specific and to develop in the majority of cases in a relatively early phase of infection. ASL is, therefore, one of the most suitable indicators for the streptococcal infection, although it is not indicative of the immune status of man (McCarty, 1954; Noguchiet al., 1962; Kusamaet al., 1962a) . One of the authors (Kusama, 1958) described a method for titrating ASL in human sera which is accurate and wellsuited for serial analyses, and motivated the initiation of the present study, which deals with close examinations of the pattern of the antibody distribution in different age groups. The extent of streptococcal prevalence was thus inferred from the percentage of those who had developed the antibody in a given population, and the difference in the capacity of antibody production by age was investigated.
