C3a, along with C4a and C5a, is a 77 amino acid anaphylotoxin generated by proteolytic cleavage during activation of the complement pathway (Ember and Hugli, 1997). The anaphylotoxins strongly promote inflammation by recruiting leukocytes, particularly basophils, eosinophils, neutrophils and monocytes (Martin et al., 1997). The proinflammatory properties of C3a are mediated by interaction between the peptide and a 7-TM G-protein coupled receptor, C3aR (Crass et al., 1996). Genetic and pharmacological inhibition of C3a/C3aR interaction indicates an important role for C3aR in allergic asthma (Humbles et al., 2000; Drouin et al., 2002). C3a/C3aR also enhances the effect of SDF-1 in promoting retention of haematopoietic stem/progenitor cells within the bone marrow (Rataiczak et al., 2004). The C3aR membrane preps are crude membrane preparations made from our proprietary stable recombinant cell lines to ensure high-level of GPCR surface expression; thus, they are ideal HTS tools for screening of agonists and antagonists of C3aR. The membranes have been shown to bind to 125I-labeled C3a with a Kd of 0.15 nM. With 0.2 nM [125I]-C3a, 10 mg/well C3aR Membrane Prep yields greater than 5 fold signal-to-background ratio.