The effects of various drugs on chemotaxis of polymorphonuclear leukocytes (PMN's) in vitro and in vivo have been studied. Response of rabbit PMN's in vitro to the chemotactic factor of rabbit serum, consisting of an activated protein-protein complex of the fifth and sixth (and probably seventh) components of complement (C'), is suppressed by hydrocortisone, methyl prednisolone, and chloroquine. Drug concentrations causing 50% inhibition of chemotaxis in vitro were found to be: hydrocortisone, 2.9 x 10–4 M; methly prednisolone, 1.2 x 10–4 M; and chloroquine 8.5 x 10–6 M. The hydrocortisone effect on PMN's appeared to be irreversible, since washing of the cells did not restore their chemotactic response. 2, 4-Dinitrophenol (DNP), vitamin A, and endotoxin did not inhibit chemotaxis. Hydrocortisone and chloroquine did block serum C' activity in vitro, but only at substantially higher concentrations. Using the reversed passive Arthus reaction in guinea pigs as a model for chemotaxis in vivo, systemic treatment of animals with hydrocortisone or chloroquine inhibited development of the vasculitis. Circulating antigen and C' were fixed in vascular structures, and serum C' was not perceptibly altered. Nevertheless, PMN infiltrates failed to occur. Local administration of hydrocortisone also prevented influx of PMN's in the Arthus reaction, in spite of the fact that immune reactants were found fixed in the vascular walls. Systemic treatment of guinea pigs with DNP did not diminish the intensity of the Arthus reactions. Phagocytosis of zymosan particles by rabbit PMN's was inhibited by hydrocortisone, methyl prednisolone, and chloroquine, but not by DNP or endotoxin. The concentrations of drugs inhibitory in phagocytosis were substantially higher than those required for inhibition of chemotaxis in vitro. These findings suggest that hydrocortisone and chloroquine inhibit the inflammatory process by preventing the response of leukocytes to chemotactic stimuli.