C5a is a proinflammatory peptide generated through activation of the complement system, and is more potent than the other anaphylatoxins, C4a and C3a, in activating peripheral blood leukocytes (Gerard and Gerard, 1994). It can actively participate in the regulation of local cytokine network by stimulating production of proinflammatory cytokines (Buchner et al., 1995; Hsu et al., 1999). The proinflammatory effects of C5a are mediated through binding to a specific 7-TM chemoattractant receptor, C5aR (CD88). Binding of C5a to phagocyte C5aR induces chemotaxis, production of superoxide anions, and release of degradative enzymes. Pharmacologic or genetic disruption of C5a/C5aR interaction reduces sepsis (Riedemann et al, 2003), immune complex-induced lung disease (Shushakova et al, 2002), and Arthrogen-induced arthritis (Grant et al, 2002) in experimental models. Cloned human C5a receptor membrane preparations are ideal tools for screening for antagonists of C5a/C5aR interactions.