The activity of the natural haemolytic complement of plasma was assayed according to the slightiy modified method using rabbit red blood cells (RRBC) as targets [31]. Equal volumes of RRBC suspension (6%) in phenol red free Hank's buffer (HBSS) containing Mg and EGTA were mixed with serially diluted plasma yo give final plasma concentrations ranging from 10% to 0.078%. After incubating for 90 min at 22 ํC, the samples were centrifuged at 400 g for 5 min at 4 ํC to avoid unlysed erythrocytes. The haemoglobin content of the supernatants was assessed by measuring the absorbancw at 550 nm in a microplate reader. The values of maximum (100%) and minimum (spontaneous) haemolysis were obtained by adding 100 of distilled water to 100 samples of RRBC, respectively. The degree of haemolysis (Y) was estimated and the lysis curve for each specimen was obtained by plotting Y/(1-Y) against the volume of plasma added (ml) on a log 10-log10 scaled graph. The volume of plasma producing a 50% alternative complement haemolysis (ACH50) was determined and the number of ACH 50 unit/ml obtained for each eperimental group.