Listeria monocytogenes is an invasive bacterial pathogen capable of multiplying inside many host cells, including macrophages, enterocytes and hepatocytes. There is evidence to believe that secretion of listeriolysin O, an SH-activated exotoxin, is crucial for bacterial growth in host tissues. This exotoxin is stimulated in iron-deprived medium and mostly active at low pH (5.5). Electron microscopic studies showed that intracellular bacteria rapidly disrupt the vacuole membrane of phagosomes and freely multiply inside the cytosol of infected cells, thus escaping at an early stage of infection from the cellular microbicidal mechanisms. Vacuole disruption does not occur with a nonhemolytic mutant obtained by insertion of a single copy of transposon Tn1545 in the structural gene of listeriolysin O. These results strongly suggest that listeriolysin O is a major factor promoting intracellular growth of L. monocytogenes and that intracellular growth of virulent bacteria is initiated after escaping from the phagosomal compartment.