An in vitro technique is described for assessing the chemotactic activity of soluble substances on motile cells. Antibody-antigen mixtures when incubated (37 degrees 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 incub...