An in vitro technique is described for assessing the chemotactic activity of soluble substances on motile cells. Antibody-antigen mixtures when incubated (37°C) in medium containing fresh (i.e. non-inactivated) normal rabbit serum exert a strong chemotactic effect on rabbit polymorphonuclear leucocytes. Results are described which indicate that, when antibody-antigen complexes are incubated (37°C) in fresh serum, a heat-stable (56°C) substance (or substances) is produced which 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 which has been heated at 56°C for 30 minutes.