Cystatin C (CC) is a cysteine proteinase inhibitor with a molecular weight of 13.000 Dalton and is formed by all nucleated cells. It is formed at a constant rate and freely filtered by the healthy kidney; it can be used for assessing renal function. Beta-2-microglobulin (Beta-2M) has a molecular weight of 12.000 Dalton and present on all nucleated cells as a component of HLA complex. It is constantly released into the blood in small quantities. Beta-2M is freely filtered and catabolized in the healthy kidney. The serum level found in healthy persons remains at a constantly low level, whereas the urine is found to contain less than 1% of Beta-2M. Beta-2M and CC are cellular products which have been shown to be good endogenous markers of GFR but only serum Beta-2M has been shown to be affected by increased cellular production. Moreover Beta-2M:CC ratio could be used as a marker for cell proliferation because the ratio should not be influence by GFR in transplant patients with normal renal functions. The study shows that increased serum concentration of Beta-2M is due to increased cellular production, but as renal function declines, this increase is due to diminish in GFR and disturbance in the catabolism, so Beta-2M:CC ratio could be used as a marker of immune activation which would be useful in adjusting immunosuppression therapy and as an adjuvant for the detection of transplant rejection.