Acidic mucopolysaccharides enhanced CĪINA activity against CĪs. The presence of heparin in the reaction mixture of CĪINA and CĪs markedly enhanced the inactivation of CĪs by potentiating CĪINA activity. Treatment of serum with CĪs resulted in the inactivation of C4 hemolytic activity, but this inactivation of C4 by CĪs was prevented by the presence of heparin presumably due to the enhancement of CĪINA activity normally present in serum. The other acidic mucopolysaccharides, chondroitin sulfates A, B and C, were also effective on potentiating CĪINA activity against CĪs, but they were less active than heparin. The enhancement of CĪINA activity by heparin was not observed in the reaction between CĪINA and plasmin.