Studies of the uptake of β-lactam antibiotics indicate that they do not accumulate in phagocytic cells. Uptake of β-lactams is thought to occur through passive diffusion, and this is limited because of their acidic nature. Many studies of antibiotic uptake have utilized either resting phagocytic cells or cells in their postphagocytic phase. We have examined the uptake of cefepime by actively phagocytosing neutrophils under various conditions of neutrophil stimulation in order to determine cefepime intracellular activity against Escherichia coli and Staphylococcus aureus. Exposure to cefepime increased bactericidal activity against E. coli both when bacteria were added daring exposure to the antibiotic and when they were added to the neutrophils in the postantibiotic phase. Antibacterial activity was only partially inhibited by phenylbutazone, and an exposure of 4 min to cefepime is sufficient for optimal intracellular activity. Under the same conditions, cefepime-associated killing of S. aureus was not as great as was observed for E. coli. Quantitation of intracellular cefepime showed that neutrophil activation in opsonizing conditions increased the antibiotic concentration by 75 (E. coli) and 55% (S. aureus). The response of neutrophils to the combination of serum, E. coli, and cefepime indicates a significant increase in the chemiluminescence response, compared with the response obtained with bacteria in the absence of cefepime. These data indicate that cefepime rapidly enters phagocytic cells under opsonizing conditions with concomitant increases in oxidative metabolism and intracellular activity against E. coli.