First part of a review describes genetic relations in a superfamily of cystein protease inhibitors: stefins, cystatins and kininogens. A protease inhibitory activity was lost in the fourth group, fetuins, which are involved in many metabolic pathways, including bone remodeling, signal transduction and inflammatory response. Cystatin C is an importem extracellular cystein protease inhibitor. Cystatin C is low molecular weight protein synthetized in a constant rate, filtered by renal glomeruli and reabsorbed and degraded by proximal tubular cells. Its plasma concentration is therefore a function of glomerular filtration rate, urine concentration and reflects tubular lesion. Plasma Cystatin C concentration correlates with standard methods of glomerular filtration rate measurements, different authors describe different level of correlation. Similarly, data of relation between Cystatin C, creatinine and creatinine clearance differ. There is a generel consensus tkat measurement of plasma Cystatin C concentration is a reliable alternative for the estimation of glomerular filtration rate in many clinical conditions.

        Key words: Cystatin C, protease inhibitors, glomerular filtration rate, creatinine.