Neutrophils (PMN) migrated in vitro chemotactically along concentration gradients of heterologous and homologous leukocytotoxic, complement-active sera. The heterologous systems consisted of normal, non-immune nurse shark or dog serum and human PMN; the homologous systems consisted of human or dog immune sera and human or dog PMN, respectively. Chemotaxis was induced 1) by fresh cytotoxic sera, 2) by complement alone if the cytotoxic antibodies were with the responding cells or if the cells were passively sensitized with the antibodies, and 3) by individual and combined complement components if the corresponding intermediate PMN-antibody-complement component complexes were the migrating cells. These observations represent a new mechanism of immune chemotaxis: the chemotactic stimulation is a direct consequence of a gradient-controlled immune reaction occuring at and involving the surface of the chemoattracted cell. This mechanism is distinctly different from that which depends on chemotactic factors produced or released independent of and at a distance from the responding cells.