Chemotactic factors released at foci of injury and various types of inflammation are thought to mediate directed migration of leukocytes into tissues. Since the identity of the infiltrating leukocyte and degree of inflammatory infiltration depend on the nature of the stimulus and on the temporal stage of the lesion, locally produced chemoattractants may act on one or more types of leukocytes (Ryan, 1967). A diversity of leukocyte chemotactic factors with differing target cell specificities have been identified (reviewed in Verghese and Snyderman, 1989). For example, activation of the complement cascade generates the C5a component of complement which is a chemoattractant for both neutrophils and monocytes (Verghese and Snyderman, 1989). In contrast, during the 1970s and in the early 1980s a number of leukocyte-derived chemoattractant activities that were selective for neutrophils or monocytes were described (Verghese and Snyderman, 1989; Tono-aka et al., 1980; Altman et al., 1973). Indeed, such cytokines generated in the course of cellular immune responses that act only on specific types of leukocytes are likely to be particularly relevant chemoattractants. However, these cytokines could not be isolated and biochemically characterized by the techniques available at that time. In the early 1980s several partially purified cytokines, such as interleukin 1 (IL 1) and tumor necrosis factor (TNF) were reported to have considerable leukocyte chemotactic activity (Luger et al., 1983; Sauder et al., 1984; Ming et al., 1987). However, when purified natural IL la/P, recombinant IL la/p, or recombinant TNFa were tested, they did not show in vitro neutrophil chemotactic activity (Yoshimura et al., 1987a). Moreover, the neutrophil chemotactic activity present in LPS-stimulated human mononuclear cell culture supernatants could be dissociated from IL 1 and TNF on gel filtration using high performance liquid chromatography (HPLC)(Yoshimura et al., 1987a). This approach