As shown previously, immune complexes engender in rabbit serum a factor capable of inducing chemotaxis of rabbit polymorphonuclear leukocytes. This chemotactic factor consists of a complex of the fifth, sixth, and seventh components of complement. As demonstrated here, the polymorphonuclear leukocytes incubated with such treated rabbit serum lose their ability to respond chemotactically to the chemotactic factor. They are "deactivated." The process of "deactivation" is a function of the duration of contact of the cells with, and the concentration of, the treated serum. There is a parallelism between the time course of deactivation and of chemotaxis, as well as the dose-response curves for the two processes. Chemotactic factor purified by isoelectric precipitation and ion-exchange chromatography produces deactivation in the same manner as the treated serum. The deactivating activity requires, as does the chemotactic factor, the sixth component of complement; like the chemotactic factor, it is heat-stable and nondialyzable. Deactivation is prevented by the same phosphonate esters shown previously to prevent chemotaxis by the complement-associated chemotactic factor. The profiles of the phosphonates in protecting against deactivation are the same as the profiles for the chemotactic factor-dependent inhibition of chemotaxis. Aromatic amino acid derivatives prevent both chemotaxis and deactivation. We conclude from this evidence that the chemotactic factor is able to deactivate or induce chemotaxis depending upon experimental conditions. The fact that the profiles given by the phosphonates for protection against chemotactic factor-dependent deactivation and for chemotactic factor-dependent inhibition of chemotaxis are the same indicates that the "activatable esterase" is involved in both processes. Acetate esters such as ethyl acetate and others shown previously to prevent chemotaxis by inhibiting the "activated esterase" do not prevent deactivation. This indicates that deactivation can occur without participation of the latter enzyme, implying that deactivation involves only a part of the biochemical mechanism of chemotaxis. The protection against deactivation afforded by aromatic amino acid derivatives is specific, insofar as nonaromatic amino compounds and simple acetate esters have no effect. In addition, as stated, the aromatic amino acid derivatives inhibit deactivation and chemotaxis by the chemotactic factor. This latter finding, together with the demonstration of the involvement of the activatable esterase in both deactivation and chemotaxis, suggests that the activatable esterase of the rabbit polymorphonuclear leukocyte is a serine esterase with a special affinity for aromatic amino acid derivatives.