The monoclonal antibody 60.11 recognizes a cell membrane C1q binding molecule that recognises theglobular heads of C1q. It is also present in plasma and the extracellular matrix. The molecule is anunusually acidic, single chain protein with an apparent molecular weight of 33 kDa. It does not possessa conventional sequence motif compatible with a membrane spanning segment nor a consensus site fora GPI anchor. gC1q-R migrates as a single chain under both reducing and non-reducing conditions, butit behaves as an oligomer on gel-filtration in non-dissociating conditions. Its multimer formation may bea critical process by which the gC1q-R molecule increases its affinity for multivalent ligands such asC1q.gC1q-R has been shown to inhibit complement activation by preventing the binding of C1q toantibodies, suggesting that the binding site for gC1q-R and the binding site for immune complexes,which are present on the C1q globular ‘heads’, may be located at the same position. gC1q-R is capableof interacting with several proteins involved in blood clotting, namely, thrombin, prothrombin, theheparinbinding form of vitronectin, the ternary complex, vitronectin-thrombin-antithrombin, as well ashigh-molecular-weight kininogen and Hageman factor. Besides its role in the complement pathway,gC1q-R participates in enhancement of Fc-receptor and CR1-mediated phagocytosis, procoagulantactivity on platelets, and chemotactic activity on mast cells, eosinophils, neutrophils, and fibroblasts.gC1q-R is expressed on a wide variety of cells and can serve as a binding site for plasma and microbialproteins. Its antigenic sites may be cryptic on cells in suspension but become exposed when the cellsare fixed by bifunctional cross-linkers. Probably it is also expressed on the cell membrane as a tetramer.Crosslinking or activation may thus bring about a tetrameric assembly of gC1q-R followed by aconformational change within the molecule, thereby exposing epitopes which are otherwise hidden. Aform of GC1q-R is also found inside the cell. Intracellular gC1q-R has been shown to bind thecytoplasmic tail of the α1B-adrenergic receptor and to PKCμ.The monoclonal antibody 60.11 is directed against epitopes situated within the NH2-terminal stretch ofgC1q-R corresponding to residues 76-93. Clone 60.11 recognizes the putative C1q binding site andreacts with the mature form, but has poor or no reactivity with the truncated form, lacking residues 74-95.