The protein adsorptive properties of monosodium urate monohydrate (MSU), silica (Si), and calcium pyrophosphate dihydrate crystals were studied by qualitative and quantitative techniques. Immunoglobulin G (IgG) was adsorbed preferentially by MSU crystals from normal human serum and demonstrated high-affinity binding isotherms when compared with several isolated proteins in solution. The physical characteristics of this reaction suggest principally an ionic mechanism, since adsorption was enhanced by decreasing pH or ionic strength. Weaker physical forces also were suggested by studies showing enhanced adsorption at lower temperatures. The following order of affinity for Si or MSU crystals was found when equal concentrations of proteins were compared: Cohn fraction II greater than lysozyme greater than beta lactoglobulin greater than bovine serum albumin greater than ovalbumin. IgG adsorption to the crystals studied may explain certain features of their biological activity. It is suggested that this phenomenon blocks the membranolytic properties of crystals and stimulates their phagocytosis through interaction with Fc receptors on the surface of the phagocytic cells.