Cystatin C, a 13 kDa-protein, is produced by most nucleated cells and is catabolized by the renal tubular cells after passing the glomerular filter. It belongs to the family 2 of the cystatin superfamily of proteins. The function of cystatin C is to regulate the activity of cysteine proteinases and cystatin C seems to be the main cysteine proteinase inhibitor of most investigated human biological fluids. Its normal level in plasma is 0.8-2.5 mg/l, in cerebrospinal fluid 4-14 mg/l and in urine 0.03-0.3 mg/l. The production rate of cystatin C is remarkably constant and its plasma concentration can therefore be used as a reliable measure of the glomerular filtration rate (GFR). Indeed, the cystatin C plasma concentration is more closely correlated to the GFR than the plasma levels of creatinine and all other investigated low molecular weight proteins, including beta 2-microglobulin and retinol binding protein. Protein HC, alias alpha 1-microglobulin, is produced by the liver as a 27 kDa-glycoprotein. It belongs to the lipocalin superfamily of hydrophobic ligand binding proteins and more than 50% of the normal plasma amount of protein HC is present as a high molecular weight HC-IgA complex carrying antibody activity. The plasma concentration of free protein HC is, in contrast to that of HC-IgA, mainly determined by the GFR. The normal values for the plasma concentrations of HC-IgA and free protein HC are 36-620 mg/l and 14-26 mg/l, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)