The ability of normal human eosinophils to ingest and kill Staphylococcus aureus and Escherichia coli was investigated and compared with the reactions shown by neutrophils from the same donors. The rate of phagocytosis of S. aureus by eosinophils was 50% of that shown by neutrophils. Unlike neutrophils, eosinophils were not able to kill ingested S. aureus at low bacterium/phagocyte ratios. The degree of S. aureus killing increased with increasing ratios, being equal to that of neutrophils when bacterium/phagocyte ratios of about 15 were used. This was probably due to a better triggering of the eosinophil oxidase system at high bacterium/phagocyte ratios. The early kinetics of the association of bacteria with eosinophils, the perforation of the bacterial envelope and the inactivation of bacterial proteins, was monitored in the ML-35 mutant strain of E. coli. The association of E. coli with eosinophils was 70% of that with neutrophils. Eosinophils had only 25% of the capacity of neutrophils to perforate the E. coli envelope. E. coli loses its colony-forming ability when the bacterial envelope has been perforated, indicating that eosinophils also kill E. coli more slowly than do neutrophils. This was confirmed with a plating assay for colony formation. The perforation of E. coli is independent of peroxidase-mediated reactions. Hence, the defective bactericidal action of eosinophils is probably not related to the differences between myeloperoxidase and eosinophil peroxidase. On the other hand, the inactivation of bacterial proteins is peroxidase dependent and was also seen to occur to a lesser extent in eosinophils compared with neutrophils. We conclude that eosinophils ingest E. coli but only slowly perforate (kill) these bacteria and barely inactivate the bacterial enzymes. In contrast, neutrophils quickly ingest and perforate (kill) E. coli and quickly inactivate the bacterial enzymes.