The adsorption of IgG to microcrystalline MSU was studied to determine its potential biologic importance in gouty inflammation. IgG showed high-affinity binding isotherms, suggesting monomolecular adsorption at concentrations of IgG found in inflammatory synovial effusions and possible surface aggregation at higher concentrations. Crystals previously exposed to a variety of other proteins showed no reduction in IgG binding or a modest one. And co-incubation of IgG with other proteins reduced IgG adsorption to only a modest extent. Highly anionic substances such as hyaluronate or PVPNO enhanced IgG binding to crystals. Studies of the functional orientation of adsorbed IgG, by using molecular probes, indicated that the Fc portion of the molecule was fully exposed, suggesting that the sites of attachment to crystal surface residue on the Fab moiety of IgG.