Some aspects of the early development of tuberculous infection were compared in vaccinated and nonvaccinated guinea pigs after low dose, respiratory challenge with Mycobacterium tuberculosis H37Rv. Changes with time of the logarithm of the number of viable mycobacteria recovered from the lungs of animals from both groups sacrificed at several intervals between zero and 35 days after challenge described a sigmoid curve with three phases: a lag phase, a phase of logarithmic growth, and a stationary or decline phase. The influence of vaccination on the rate of accumulation of the challenge organism was not detected before 10 to 14 days after infection. The significance of these results is discussed with reference to the important role a critical amount of antigen can play in any test system in which acquired cellular immunity is studied.