The mechanism of the reaction between EAC1̄, and C5 has been studied by kinetic analysis of C5 consumption and formation of SAC1̄,,5b with variation of the concentration of reactants, temperature, pH and ionic strength. The results are compatible with the hypothesis that EAC1̄, enzymatically cleaves the C5 molecule into two fragments, one of which, C5b, then becomes fixed to the cells. The minimal hemolytic efficiency of purified guinea pig C5 is about 10%. As expected, the rate of C5 consumption by EAC1̄, and that of formation of SAC1̄,,5b diverge as temperature or ionic strength are allowed to vary. N-acetyl-l-tyrosylethylester blocks C5 consumption and SAC1̄,,5b formation; on the other hand, phloridzin inhibits SAC1̄, formation but not C5 consumption. Cell-bound C2 and C3 are required for consumption of C5 and these components have to be located on the same cell, probably in close proximity to one another.