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Publisher Summary In this chapter, it is shown that Fcγ or C3b receptors could be detected on human lymphoblastoid cell lines using solid-phase radioimmunoassays (RIA) specific for human IgG and human C3b, respectively. These radioimmunoassays are very simple, rapid, and sensitive; 10–10 M aggregated human IgG (AHIgG) or monomeric IgG and 10–9 M C3b are being detectable. The sensitivity of the RIA specific for human C3b is one-tenth that than the assay for human IgG owing to loss of antigenic activity of 125 I-labeled C3b of high specific radioactivity. The use of acrylamide beads as the solid phase has the advantage of high stability in systems that lack isotonic buffers or contain complement or detergents. At this level of sensitivity, FcR- or C3bR-positive cells can be detected at concentrations as low as 5 × 10 4 cells per well. Furthermore, the test is suitable for routine determinations, does not require great expertise of the investigator, and makes it possible to screen a high number of samples in triplicate in a few hours. It, thus, compares favorably with rosette tests or immunofluorescence techniques. Sensitivity of the RIA may be increased using monoclonal antibody directed against human IgG or human C3b.