A prospective study of infection with TB among 680 contacts of 187 families in which there was at least one adult with active TB was carried out by the Dept. of Internal Medicine and Medical School of Texas. The family unit was defined as the one which occupied the same domicile (nuclear and extended). Three distinct groups: 1) Spanish-speaking Americans (SSA), 2) English-speaking whites (ESW), and, 3) English-speaking Negroes (ESN) appeared in the study population. The six characteristics chosen for study were, intimacy with source, severity of disease, age of contact, income, crowding and mode of living. The first three factors were independent of social factors. The analysis was done by scoring method. The findings of the study revealed a gross rate of 47% infection. A wide range of tubercular infection existed; 26.7%, 17.6% and 11.9% of all the contacts of the families of ESW, ESN and SSA were infected respectively. Similarly, no contacts of 27% of the families were infected. Infection in less than 6 years of age was nearly the same. For the three population groups, the rate of infection of 53% was highest among the SSW and lowest among the ESN, with a rate of 42.5%. It was found that the ESW had the stronger family structure and ESN the most unstable structure. The social factors of overcrowding and an impoverished mode of living has an important effect. Rates of infection are about 2-3 times higher when sputum is infectious and is positive on microscopy examination. The non-radiological factor being associated with higher infection rate is the outstanding feature of all studies and findings of this study are consistent with the other studies. Social characteristics seem to operate only as they contribute to the environmental factor in the transmission of tubercular infection