Inflammation is a process, and not a state, of the succession of changes which occurs in a living tissue when it is injured. Each change is controlled by both activating and inhibiting substances in the local lesions, and there exist also a process to produce these substances in the loci. However, the mechanism underlying such a process has been the subject of considerable discussion. Particular interests in inflammation are whether the motile cells present in the blood are attracted to inflamed spots and if so what chemical substances produce the effect.<BR>The histological picture of granulomatous inflammation is both variable and nonspecific, but macrophages and their derived cells(epithelioid cells and multinucleated giant cells)provide a major features of various examples of the inflammation. These macrophages come from the peripheral blood monocytes which are derived from bone marrow. The granulomatous inflam mation is sustained by the persistence of the macrophages which may result from their con tinuing mobilization from the bone marrow, from their local proliferation, or from longevity of the cells in the tissues.<BR>The most attractive interpretation of the macrophage infiltration in inflammation is that a substance exists around the attracting cells in a diminishing concentration gradient and that the cells have some mechanisms which directs them to the more concentrated area (chemotaxis), but the mechanism of this phenomenon is unknown. The possible importance of macrophage chemotaxis in inflammation is highlightened by the discovery of the chemotactic factors for macrophages in the inflammatory sites. At least three factors are found in vivo sites and the relative activity of the three factors are different with each other in the type of inflamm ation. The most active factor in the delayed hypersensitivity skin sites is highly purified and it shows a single band in disc gel electrophoresis. It is a heat-labile glycoprotein with a molecular weight of 150, 000 and is different from lymphokine and C5a. One of the factors is shown to be produced by a serine-type protease of polymorphonuclear leukocytes from IgG which is permeated from the vessels in the initial stage of inflammation.<BR>Large numbers of experiments has been performed in vitro on the chemotaxis of le ukocytes using many methods, and macrophages are found to be attracted by many substances, i. e., bacterial, serum-derived (complement-derived and not), lymphocyte-derived, tissue-derived, and so on. However, the relative importance of these factors in the in vivo reaction is still far from clear. It should be emphasized that the in vivo study is very important to elucidate the mechanism of macrophage infiltration in the inflammatory sites.<BR>The whole question of chemotaxis is still premature but is an extremely active and rapidly moving area of investigation, and “attracts” many of the investigators. The delineation of amolecular basis for in vivo chemotaxis in inflammation is the focus of much current interest and warrants further interesting investigations.