The polymorphonuclear leucocytes isolated from blood or an inflammatory exudate are able to respond to chemotactic stimuli (‘tactism’). This property is readily demonstrated by the following technique1. A suspension of starch granules, isolated from potatoes, is spread on microscope slides and the films fixed by drying. A few drops of a suspension of leucocytes are placed on a slide so prepared, a cover-slip superimposed and the edges sealed with ‘Vaseline’ and wax. The preparation is then placed on the stage of a microscope in a hot-box at 37° C. Almost at once the leucocytes, attracted no doubt by soluble dextrin liberated into the medium, move towards the starch granules, and finally, within an hour, adhere to the surface of the latter in the form of regular crescents.