SUMMARY In this study we demonstrate that, in addition to blood,_α1-microglobulin (_α1m) is present in most tissues, including liver, heart, eye, kidney, lung, pancreas, and skeletal muscle. Western blotting of perfused and homogenized rat tissue supernatants revealed_α1m in its free, monomeric form and in high molecular weight forms, corresponding to the complexes fibronectin-_α1m and_α1-inhibitor-3-_α1m, which have previously been identified in plasma. The liver also contained a series of_α1m isoforms with apparent molecular masses between 40 and 50 kD. These bands did not react with anti-inter-α-inhibitor antibodies, indicating that they do not represent the_α1m-bikunin precursor protein. Similarly, the heart contained a 45-kD_α1m band and the kidney a 50-kD_α1m band. None of these_α1m isoforms was present in plasma. Immunohistochemical analysis of human tissue demonstrated granular intracellular labeling of_α1m in hepatocytes and in the proximal epithelial cells of the kidney. In addition,_α1m immunoreactivity was detected in the interstitial connective tissue of heart and lung and in the adventitia of blood vessels as well as on cell surfaces of cardiocytes._α1m mRNA was found in the liver and pancreas by polymerase chain reaction, suggesting that the protein found in other tissues is transported via the bloodstream from the production sites in liver and pancreas. The results of this study indicate that in addition to its role in plasma,_α1m may have important functions in the interstitium of several tissues.