The effect of antimicrobial agents on the intracellular multiplication of Legionella pneumophila in cultured guinea-pig peritoneal macrophages was measured. Beta-lactam antibiotics at concentrations 5 to 400 times the MIC in vitro did not inhibit the intracellular growth of the organism. Gentamicin inhibited the growth considerably but failed to eliminate the organism from the phagocytic mixture. Chloramphenicol or tetracycline at 10 micrograms ml-1 (40 or 5 times the MIC in vitro respectively) did not eliminate the organism. At a higher concentration (30 micrograms ml-1), however, these drugs eliminated the bacterium from the mixture. Only erythromycin and rifampin were effective in killing the organism at very low concentration (1 microgram ml-1). Intracellular multiplication of L. pneumophila was observed clearly by light microscopy using Wright-Giemsa staining.