When confluent monolayers of Madin-Darby Canine Kidney (MDCK) renal epithelial cells are exposed to calcium oxalate monohydrate (COM) crystals suspended in the nutrient media, crystals settle onto the cell surface. Here, they become entangled with the specialized membrane structures of cilia and microvilli (Figures 2). The cell membrane surface is structurally altered as patches of membrane devoid of microvilli are found adjacent to the crystal entrapped on the surface (Figure 4). Many COM crystals disappear from the cell surface and appear to be internalized. This phenomenon is seemingly common to all cultured renal epithelial cells. The uptake of crystals appear to increase with time and crystal concentration and stimulates a series of complex reactions. Morphologically, crystal aggregates are contained with intracellular vesicles which appear to be membrane lined (Figure 2,3). In addition, cells exocytose the internalized crystals and in MDCK cells, crystal-cytoplasm complexes can be observed leaving cells at the intercellular junctions.