The effect of brief exposure of Escherichia coli to chloramphenicol on the antibacterial activity of normal human leukocytes was studied by following changes in viability of the bacteria in the presence of leukocytes and serum. Growth was suppressed, and the extent of suppression was directly related to the period of exposure and the concentration of chloramphenicol. When exposed to clinically achievable levels of the drug for 10 min, E. coli failed to resume normal growth for 1 to 4 h in the presence of leukocytes and serum after removal of the drug. The post-antibiotic leukocyte enhancement effect required the presence of antibody and complement. This effect demonstrates the importance of early events in the encounter between antibiotic and microorganism in determining the subsequent activity of host defense components.