Hallmarks of autoimmune diseases such as SLE are hypergammaglobulinemia, the production of autoantibodies, and elevated levels of circulating immune complexes . Several mouse strains are used as models of SLE (see reference 1 for review), including NZB, NZB/NZW Ft , MRL/lpr, and BXSB . In human and murine SLE, there is frequently a .pronounced inhibition of macrophage binding and phagocytosis of IgG-sensitized erythrocytes and immune complexes (2-4) . One possible reason for this inhibition is the downmodulation of the macrophage Fcy R after binding of immune complexes . An alternate explanation for the inhibition of Fc yR function might be the binding of anti-Fcy R autoantibody to the macrophage FcyR .In this study, we have used a recombinant truncated FcyR (tFcyR) t to screen murine sera and mAbs for naturally occurring anti-Fcy R antibody. The tFcyR is derived from murine FcyRIIf, and consists of the two external Ig-like domains of muFcy RIIa, but lacks the transmembrane and cytoplasmic domains of the receptor (5) . We report here the presence and characterization of anti-Fcy R antibodies in sera ofsome mice genetically prone to autoimmune diseases and anti-Fc yR hybridomas derived from NZB and motheaten (me") mice.Volume 171 May 1990 1581-1595 AUTOIMMUNE MICE MAKE ANTI-Fcy RECEPTOR ANTIBODIES Materials and Methods mAbs and Sera. The mAbs that we screened for anti-FcyR activity were derived from fusions with spleen cells from old NZB mice or young NZB mice after stimulation with LPS (6) . Other hybridomas screened were from unstimulated viable me' mice (7) . The IgM mAbs were all purified by affinity chromatography on an anti-tc IgG-Sepharose column and were eluted from the column with 0 .1 M glycine HCI, pH 2 .3 . Samples of sera from NZB, NZB/NZW F,, MRL/lpr, and BXSB mice were kindly given to us by Dr. Argyris Theofilopoulos (Scripps Clinic, La Jolla, CA) . Sera from me', tightskin mouse (TSK), C57B1 .6pa/pa, and C58/J were obtained from animals purchased from The Jackson Laboratory (Bar Harbor, ME). Anti-DNP hybridomas DHK109 .3 (IgGl) and DHK10 .12 (IgG2b)