A new, specific hemolytic assay for C2 [the 2nd complement component] is described using EAC43 [sheep erythrocyte, antibody, C4 + C3 complex] (Antrypol) cells and serum deficient in C4 and C2 from a patient with hereditary angio-edema. The method is simple and the results in pathological and normal sera correlate well with those obtained using the EA and C2-deficient serum method.