Infections caused by intracellular organisms often involve the lung and may be implicated in chronic disease. These intracellular bacteria may be protected from otherwise lethal concentrations of extracellular antimicrobials. Knowledge of the intracellular concentration of usual antimicrobials used to treat pneumonia may allow physicians to refine their initial choice of therapy. Lipid-insoluble antimicrobials, such as penicillin, cephalosporins, aminoglycosides, trimethoprim, and imipenem penetrate poorly into cells, if they penetrate at all. Isoniazid, tetracycline, and lincomycin have intermediate intracellular penetration, and chloramphenicol, rifampin, ethambutol, quinolones, and lincosamides, plus macrolides, are avidly concentrated. Nonetheless, to date it has been difficult to correlate intracellular concentrations of antimicrobials with cellular killing or clinical outcome. Information derived from a more standardized approach to the evaluation of antimicrobial agent intracellular penetration will be useful in improving the direct application of in vitro study results to the clinical care of patients with pneumonia.