Erythromycin base and its 6-0-methyl derivative clarithromycin were actively accumulated 7.3 ± 1.2-fold and 9.2 ± 2-fold respectively by human neutrophils in vitro. The intraphagocytic bioactivities of the antimicrobial agents were investigated using the combination of a radioassay, colony counting method and a fluorescence microassay which facilitates the distinction between intracellular bacteriostatic and bactericidal mechanisms. Staphylococcus aureus, Listeria monocytogenes and Legionella micdadei were used as the test intraphagocytic microbial pathogens. Both agents were found to possess intracellular bioactivity for all three species of bacteria with clarithromycin being consistently more active than erythromycin. Under the assay conditions used both agents were bacteriostatic (intracellularly) for S. aureus and Leg. micdadei and bactericidal for List. monocylogenes. Clarithromycin is clearly a potent intraphagocytic antibiotic and potentially superior in this respect to erythromycin.