METHODS Some infants immunized with BCG in the newborn period fail to develop any measurable tuberculin reactivity despite a local reaction at the site of immunization. OBJECTIVE We wished to determine the possibility of a genetic regulation of this phenomenon by comparing the tuberculin reactivity of BCG-immunized parents and siblings of infants who failed to respond to BCG, and of infants who developed tuberculin reactivity after immunization. METHODS We studied 65 parents and siblings of 33 nonresponder infants, and 35 parents and siblings of 14 infants who had developed tuberculin reactivity. Tuberculin reactivity was analyzed by multiple regression analysis considering the BCG immunization status of each individual, and the 2 groups were compared by analysis of covariance. RESULTS 96 of these family members had one or more BCG scars. The percentages of tuberculin reactors and non-reactors among BCG-immunized family members of both index infant groups were not significantly different. CONCLUSIONS These observations suggest that maturational differences among newborns, rather than genetic regulation, account for the lack of development of cellular immunity against tuberculin after BCG immunization in some infants.