Cells from a human lymphoblastoid cell line (Raji), with B-cell characteristics, and having receptors for human IgG Fc, C3b, and C3d, were used in an immunofluorescence test as in vitro detectors of immune complexes in animal and human sera. By this test, as little as 200–300 ng aggregated human gamma globulin or immune complexes per ml serum could be detected. The receptors for IgG Fc on the Raji cells were shown to be inefficient in detecting aggregated human gamma globulin and binding of aggregates to these receptors was inhibited by physiologic concentrations of 7S human IgG. Enhancement of aggregated human gamma globulin binding and binding of immune complexes formed in vitro to Raji cells was observed when the receptors for complement on these cells were used. By using the receptors for complement on Raji cells, circulating immune complexes were detected in rabbits with acute serum sickness, in mice with acute lymphocytic choriomeningitis virus infection, and in humans with immune complex type glomerulonephritis. The Raji cell test may be useful in detecting complement fixing immune complexes in different disease states, in monitoring circulating complexes in patients with immune complex diseases and in identifying the antigen(s) responsible for the induction of pathogenic immune complexes in humans and animals.