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The alpha 1 beta 1-integrin is known to be a receptor for collagen and laminin mediating cell-matrix interactions. A monoclonal antibody, 33.4, which specifically inhibits the alpha 1-integrin-mediated in vitro cell-collagen binding of rat hepatocytes and hepatoma-derived A-cells (Löster et al., 1994), was used to purify by immunoaffinity chromatography the alpha 1-integrin subunit from rat liver in large quantities for inducing a polyclonal antiserum. In immunoblot analysis on membrane extracts of several rat organs this polyclonal antiserum recognized only a 190 kDa-band, suggesting that it is highly specific for the alpha 1-integrin subunit. A sandwich-ELISA with monoclonal antibody 33.4 and the polyclonal antiserum against the alpha 1-integrin subunit, respectively, enabled the quantitative expression pattern of the alpha 1-integrin subunit to be studied in different rat organs. With the exceptions of brain (not detectable) and muscle (low concentration), the alpha 1-integrin subunit was detectable in almost all organs of the digestive, respiratory and urogenital system as well as in lymphatic organs. The highest relative concentrations of alpha 1-integrin subunit were found in uterus, lung and spleen, whereas in seminal vesicle, stomach, parotid gland, epididymis, kidney and liver only modest concentrations were evident. The organ distribution and localization of alpha 1-integrin subunit were studied by immunohistochemistry with monoclonal and polyclonal antibodies. Immunoreactivity was present in the plasma membranes of all smooth muscle cells, vascular endothelial cells of many organs and fibrocyte-fibroblast sheaths in the heart and kidney. Since these cells are in close contact with collagen-containing basal membranes as well as reticular fibrils, strong evidence exists that in rat tissues the alpha 1-integrin subunit is expressed at sites where collagen is present and might be involved in vivo in cell-collagen binding.