Alveolar macrophages from the lungs of guinea-pigs that had been vaccinated, boosted and then intravenously challenged with Mycobacterium microti or Mycobacterium bovis BCG killed tubercle bacilli phagocytosed in vitro. The killing was modest, about 40% of phagocytosed bacilli were killed in a day, but alveolar macrophages from animals that had been vaccinated and boosted but had not received the intravenous challenge did not kill bacilli. Different strains of tubercle bacilli had different degrees of susceptibility to these activated macrophages but there was no correlation between killing by macrophages and mycobacterial susceptibility to killing by hydrogen peroxide. The different strains of tubercle bacilli triggered peroxide release from these macrophages but there was no correlation with susceptibility to killing by macrophages or with virulence in the guinea-pig. However, phagocytic uptake of these strains by the activated macrophages was inversely correlated with virulence, and uptake by activated macrophages was less than uptake by normal macrophages.