Studies were undertaken to demonstrate the efficacy of erythromycin in killing Legionella pneumophila organisms that have been ingested by human polymorphonuclear leukocytes. With use of an electron microscope-autoradiography method in conjunction with an antibiotic-uptake assay, it was shown that [3H]-labeled erythromycin A base penetrated into the cytoplasm of human polymorphonuclear leukocytes in concentrations up to 24 times higher than the extracellular drug level. Similar studies using [14C]benzyl penicillin revealed that this antibiotic penetrated human polymorphonuclear leukocytes very poorly. Removal of erythromycin from the extracellular environment resulted in rapid leakage of radioactivity from the cells; an assay for bioactivity showed that this material was unaltered drug. Human polymorphonuclear leukocytes were permitted to ingest L. pneumophila and were then treated with erythromycin A base. The erythromycin killed the ingested legionellae, as evidenced by bacterial plate counts and morphological destruction of ingested organisms. These data clearly demonstrate the efficacy of erythromycin in killing susceptible intracellular pathogens.