Publisher Summary An in vitro technique is described for assessing the chemotactic activity of soluble substances on motile cells. Antibody–antigen mixtures when incubated in medium, containing fresh, that is, noninactivated normal rabbit serum, exert a strong chemotactic effect on rabbit polymorphonuclear leucocytes. Results are described that indicate that, when antibody–antigen complexes are incubated in fresh serum, a heat-stable is produced that acts directly as a chemotactic stimulus on the polymorphs. This heat-stable chemotactic substance is not produced when antibody–antigen complexes are incubated in serum that has been heated for 30 minutes. It is reasonable to suppose that antibody–antigen complexes can exert a chemotactic effect in vivo as well as in vitro. This has obvious implications in relation to specific acquired resistance to natural microbial infections; it may also offer an explanation for the massive polymorphonuclear infiltration that follows intradermal injection of antigen in the Arthus reaction. It would be worthwhile to try to identify the chemotactic factors that influence these cells, perhaps taking as a basis for research Pulvertaft's demonstration of the extraordinary interest that lymphocytes show in cells that are undergoing mitosis.