Background: It has been shown that antibacterial agents affect polymorphonuclear leukocytes (PMNs), which are active in the body's first line of defense, in different ways. However, few studies have investigated the effects of tablet dosage forms as opposed to pure powder forms. There is a need to demonstrate the clinical relevance of previous results with commercially available products. Methods: We examined the effects of clarithromycin solutions, prepared separately from either pure powder or commercially available tablets (250 mg), on human PMNs. The in vitro effects of each solution, adjusted to therapeutic concentration (1 mg/l), on PMN adherence, chemotaxis, phagocytosis, candidacidal capacity and superoxide production were studied. Results: Solutions prepared from pure clarithromycin powder did not affect the adherence, phagocytosis or superoxide production of PMNs, but did inhibit (p < 0.05) chemotaxis and candidacidal capacity. By contrast, a decrease (p < 0.05) in all functions except phagocytosis was observed with solutions prepared from the tablet dosage form of clarithromycin. Conclusion: The results of this study suggest that the tablet dosage form of clarithromycin may have a more pronounced inhibitory effect on human PMN functions than solutions prepared from the pure powder form. Copyright (C) 2000 S. Karger AG, Basel.