Exposure of human polymorphonuclear leukocytes (PMN) to chemotactic factor, as well as the migration of PMN through a 5-mum pore-size membrane, results in a PMN population with enhanced chemiluminescence, enhanced capacity for superoxide anion production, and increased Escherichia coli bactericidal activity. The enhanced PMN response resulting from exposure to chemotactic factor was observed with several chemotactic stimuli, including a mixture of casein and autologous serum, chemotactic C5 fragment, and formyl-l-methionyl-l-leucine-l-phenylalanine (f-Met-Leu-Phe). Enhanced levels of chemiluminescence were observed with both soluble stimuli (concanavalin A and phorbol myristate acetate) as well as particulate stimuli (opsonized zymosan). Once activated by chemotactic factor, PMN retained their enhanced stimulated chemiluminescence in the absence of chemotactic factor for at least 2.5 h. Enhanced activity could not be correlated with a shift in the number of immunoglobulin (Ig)G Fc receptor positive or complement receptor positive PMN. In vivo studies with guinea pigs indicated that PMN attracted to an intraperitoneal injection of casein, like those attracted through a chemotaxis membrane in vitro in response to casein, showed markedly enhanced stimulated chemiluminescence when compared with peripheral blood PMN from the same animal. Such a mechanism to stimulated PMN function may enhance the effectiveness of PMN in host defense at inflammatory foci.