Cell-CAM 105 is involved in intercellular adhesion of isolated rat hepatocytes in vitro. In addition to liver, cell-CAM 105 occurs in several different epithelia, in platelets, and in granulocytes. In this paper we present a detailed analysis of cell-CAM 105 in the small intestinal mucosa. Isolated rat intestinal epithelial cells and brush-borders were investigated by indirect immunofluorescence. A strong fluorescence occurred in the brush-border region and a much weaker staining was seen in the lateral cell surfaces. The brush-border staining was heterogeneous and concentrated to the periphery where brush-border microvilli from adjacent cells are in contact with each other. It is suggested that cell-CAM 105 might mediate binding between the outer surfaces of neighboring microvilli. Immunoblotting and electrophoretic analyses of the biochemical properties of intestinal cell-CAM 105 demonstrated significant differences compared with cell-CAM 105 isolated from liver. Intestinal cell-CAM 105 was smaller than liver cell-CAM 105 under reducing conditions, whereas it was larger than liver cell-CAM 105 under non-reducing conditions. Chemical reduction decreased the size of intestinal cell-CAM 105, but increased the size of liver cell-CAM 105. Our interpretation of these data is that intestinal cell-CAM 105 occurs as a part of a macromolecular complex. This interpretation was supported by electrophoretic analyses of intestinal cell-CAM 105 isolated by immunoaffinity chromatography on anti-cell-CAM antibodies. In addition to cell-CAM 105, this material contained several other proteins of lower molecular weight than cell-CAM 105. These data suggest that intestinal cell-CAM 105 participates in cell-surface interactions that may regulate the structure and function of the apical brush-border regions of the intestinal epithelial cells.