Chemotaxis is defined as the ability of living cells to determine the direction of their locomotion along a concentration gradient of attractant and repellant substances. A survey is given of the various in vitro techniques for detecting chemotaxis in leukocytes including monocyte macrophages, polymorphonuclear cells, lymphocytes and bacterial cells. Biological substances such as complement and its components, factors derived from lymphocytes and other proteins which cause chemotaxis are also described. Finally, reference is made to the relationship between chemotaxis and manifestations of disease (e.g. inflammatory response) and defects in the chemotaxis-inducting systems in man in certain diseases (Chediak-Higashi syndrome, lazy leukocyte syndrome, etc.). Possible means of therapy of abnormal chemotaxis in man are still in the discussion stage.