Methods of measuring the activity of the total alternative complement pathway, properdin and Factor B with rabbit red blood cells(RaRBC) were examined. The following results were obtained. 1) 0.03M EGTA-GVB permitted activation of the alternative complement pathway without activation of the classical complement pathway. 2) RaRBC were hemolysed by C4/C2 depleted serum as well as by normal serum. RaRBC were not hemolysed by inulin or zymosan treated serum. 3) The activity of the total alternative complement pathway was measured with 50 % hemolysis of RaRBC in EGTA-GVB, designated as ACH50. 4) The activity of properdin or Factor B was measured with RaRBC and normal fresh human serum deficient in properdin (RP) or deficient in Factor B (RB).