The lymphocyte stimulation test was used to estimate specific cell-mediated immunity after vaccination with the live vaccine strain of Francisella tularensis. Nonvaccinated individuals and individuals vaccinated 1, 5 to 6, 7 to 8, and 9 years previously were tested. Lymphocytes from most vaccinees responded to an antigen preparation of the vaccine strain, and those vaccinated 9 years before testing responded to a similar extent as did vaccines in the other groups. A new technique was developed to study the presence of T lymphocytes among the stimulated cells. Stimulated cells were allowed to incorporate [14C]thymidine and were then fractionated into T and non-T lymphocytes. Most of the incorporation appeared in the fraction containing T lymphocytes. The data indicate that cell-mediated immunity endures for at least 9 years after vaccination with the live F. tularensis vaccine.