The following aspects of random and directional migration of leukocytes have been considered: (1) basic cellular mechanisms involved in leukocyte locomotion; (2) the mechanisms of formation and the nature of various factors such as cytotaxins, cytotaxin-inactivators and inhibitors participating in the regulation of the chemotactic response, and (3) the relationship between chemotaxis and leukocyte emigration. The need for adequate techniques for measuring chemotaxis has been emphasized with special reference to clinical investigation.