The correlation of the level of acquired. resistance of vaccinated and unvaccinated guinea pigs to aerogenic challenge with small numbers of Mycobacterium tuberculosis H37Rv with the degree of hypersensitivity to tuberculin measured at the time of challenge was studied. Under certain conditions of vaccination, there was a significant inverse correlation between the erythematous response to 100 tuberculin units of purified protein derivative and the number of M. tuberculosis H37Rv recovered from the lungs four weeks after aerogenic challenge. This correlation was found both six weeks and six months after vaccination with bacille Calmette Guerin. There was no correlation if 5 tuberculin units of purified protein derivative were used for skin testing. Hypersensitivity to tuberculin waned with time, but acquired resistance did not. In view of these results, it is hypothesized that immunologic memory is the basic requirement for acquired resistance rather than the presence of circulating sensitive lymphocytes detected by the skin test.