Mindful of the current widespread use of antibiotics in the treatment of infectious disease and of the consequent significant alteration of the bacterial population of the environment, concern has been felt by the authors regarding possible interference with normal human immune responses and therefore ultimately with resistance to the very diseases being treated. Group A hemolytic streptococci were chosen for this study because they are easily isolated and can be precisely identified. Furthermore, the longchain test previously described by Stollerman for determining anti-M protein antibodies provides a means of measuring the type-specific immunity of a population, which can then be related to past experience. Using data amassed in over a decade of streptococcal research in this laboratory, an attempt has been made in this paper to determine the length of survival of anti-M protein antibodies in the studied population in order to provide a means of measuring type-specific streptococcal immunity in the group. The method of culturing families, collecting sera, calculating the index, and recording and interpreting data is described in detail and summarized in graphs and charts which follow the text. Among the significant results of this study was the demonstration of the importance of the untreated carrier state in the development and persistence of antibodies. The influence of penicillin was obvious. Data on persons ill with streptococcus, who were given the antibiotic, and in whom no late carrier period was observed showed low long-chain indices with only six exceptions, these all from known carrier families who were not available for culture at the critical months. High values were found for as long as 8 years after the first known positive culture, with, however, a pronounced dip at 3 years and subsequent high values in later years. It is suggested that the frequent reexposure of normal childhood would trigger the recall mechanism and result in secondarily elevated titers. Three of the data tables suggest that the long-chain index may indeed be a measure of type-specific immunity, an idea originally reported by Stollerman. Twenty-four persons from whom early sera had been obtained, and who became ill, all had negative titers, and the nine others who did not become carriers after illness had low titers when tested at 2 years. Of the 13 persons known to have had experience with the organisms, none became ill, and all whose sera could be tested had elevated titers. These findings suggest that in an undisturbed natural setting a fluctuating distribution of types in different years makes possible the repeated stimuli which trigger the recall mechanism, and in turn that a fluctuating pattern of anti-M protein antibodies in potential hosts makes possible the reestablishment of a particular type in a community after an absence of several years. The fact that the group C and G streptococci showed no particular variation confirms the theory that anti-M antibodies and the prevailing types in a community are interrelated: the nonpathogenic, nonantigenic strains were not influenced by differences in host receptivity. Conversely, the prolonged absence of typespecific antigens, such as occurs after interference with natural host-pathogen relationships, would permit titers to fall to the point at which susceptibility to illness would increase. It would seem then that the asymptomatic, untreated carriers serve an important purpose in maintaining type-specific immunity in a population.