In vivo cell interactions between Staphylococcus aureus and rat alveolar macrophages were investigated after intranasal inoculation during a 30-days period of examination. Some dynamic characteristics of microorganisms in the macrophages were examined by electron microscopy and acid phosphatase cytochemistry. It was found that at earlier infection intervals (days 3 and 7) the ingested cocci were sequestered in phagosomes and phagolysosomes and later many of the microbial cells were digested. An interesting finding was the intracellular appearance of cell wall-defective forms (L-forms) of S. aureus at later intervals (days 14 and 30 after challenge). Infection kinetics were evaluated by isolation and enumeration of colony-forming units of S. aureus from bronchoalveolar fluid and by assessment of blood and bronchoalveolar total and differential leukocyte counts. The results indicate that induction and survival of S. aureus L-forms may occur spontaneously in vivo. This phenomenon could explain some of the mechanisms, provoking the latent and relapsing lung infections.