A radioimmune assay for the detection and quantitation of circulating immune complexes has been developed which employs the L1210 murine leukemia cell. The assay is based upon the binding of immune complexes to the L1210 through its Fc receptor followed by quantitation of the complexes with an 125I-labelled anti-IgG. The radioactivity of the cell pellet is referred to a standard curve generated by incubating the L1210 with known amounts of heat aggregated IgG (AIgG). 7S IgG of three species (human, canine, murine) do not bind significantly to the L1210 in contrast to the respective AIgG. The assay readily distinguishes between sera of healthy individuals and sera of individuals (human and canine) with diseases known to be associated with circulation immune complexes (i.e., systemic lupus erythematosus, HBAg positive acute hepatitis). The L1210 radioimmune assay is capable of detecting as little as 5 micrograms of immune complexes per ml of serum in all three species tested. The assay possesses several advantages over those currently employed, the most notable being the ability of the L1210 cell to detect immune complexes irrespective of their complement fixing properties.