α1-Microglobulin (protein HC) is a lipocalin with immunosuppressive properties. The lipocalins are a family of proteins which transport small hydrophobic molecules such as steroids, bilins, retinoids, and lipids. Well known lipocalins are α1-microglobulin, alpha-1-acid glycoprotein (orosomucoid), apolipoprotein D, beta-lactoglobulin, and complement component C8 gamma chain etc. A gene in the lipocalin cluster encodes α1-microglobulin together with a Kunitz-type proteinase inhibitor, bikunin. The gene is translated into the α1-microglobulinbikunin precursor, which is subsequently cleaved and the two proteins secreted to the blood separately. α1-Microglobulin, is a monomeric protein of a molecular size of 27 kDa. It is produced by the liver and circulates in a free form, but is also bound in complexes to high molecular weight molecules. In human plasma, approx. 50% of α1– microglobulin forms a one-to-one complex with monomeric immunoglobulin A (IgA) by a reduction resistant bond. Normal urine contains very small amounts of α1– microglobulin. In conditions with disturbed tubular function, reabsorption of α1– microglobulin is reduced and increased amounts are found in urine. Urinary α1-microglobulin seems a promising marker for renal abnormalities and for the differentiation of various nephrological and urological pathologies.