The inhibitory and postantibacterial activities of pefloxacin, ciprofloxacin, and ofloxacin against virulent Legionella pneumophila serogroup 1 were evaluated in cell-free and cellular models. In the absence of macrophages (with the tissue culture medium alone), bacterial numbers remained unchanged at 24 h in the presence of 0.1 microgram of pefloxacin, ciprofloxacin, or ofloxacin per ml and 1.0 microgram of pefloxacin per ml, whereas they were reduced in the presence of 1.0 microgram of ciprofloxacin or ofloxacin per ml. Experiments to evaluate the postantibacterial effects of these drugs were therefore performed with concentrations of 0.1 microgram/ml. In the cell-free model, brief exposure (1 h) of bacteria to each antimicrobial agent resulted in a transient decrease in numbers followed by logarithmic growth. In the cellular model, all three drugs (at 0.1 and 1.0 microgram/ml) inhibited the intracellular multiplication of L. pneumophila. The intracellular postantibacterial effects of 0.1 microgram of pefloxacin, ciprofloxacin, and ofloxacin per ml, which were left in contact with L. pneumophila-infected human macrophages for 24 h, were evaluated at various times after removal of the drugs. Pefloxacin was found to exhibit a significant inhibitory effect at 72 h, whereas following the removal of ciprofloxacin and ofloxacin, rapid bacterial multiplication occurred, leading to the destruction of the macrophage monolayer within 48 h. Thus, while pefloxacin, ciprofloxacin, and ofloxacin all inhibited the multiplication of L. pneumophila in human monocyte-derived macrophages, only pefloxacin exhibited a prolonged postantibacterial effect.