alpha 1-Microglobulin (protein HC) is a stable urinary indicator protein which reflects acute and chronic dysfunctions of the proximal renal tubule. High urinary concentrations were found to be indicative of tubular drug toxicity, interstitial nephritis or chronic renal failure. The protein is synthesized by liver cells and readily associates with serum immunoglobulin A. Only the free form is filtered through the glomerulus and is reabsorbed by proximal tubular cells. The exact physiological function of this member of the new superfamily of lipocalins is still unknown, but there are indications that alpha 1-microglobulin (protein HC) may serve as an excretion vehicle for fluorescent, charge-heterogeneous substances of unknown nature. Additionally, it seems to be associated with the humoral and cellular immune response.