A macromolecular complex containing guinea pig complement fragment C5b (activated C5) and component C6 has been obtained from ĒĀC̄1̄,4̄b̄,2̄ā,3̄b̄,5̄b̄,6̄ (sheep erythrocytes carrying antibody and complement components, or fragments, C1, C4b, C2a, C3b, C5b and C6) or from ĒĀC̄4̄b̄,2̄ā,3̄b̄,5̄b̄,6̄ by elution. This complex combines with EAC4b,3b to yield an intermediate termed pseudo-EAC-6, which can be lysed by C7, C8 and C9, like genuine EAC-6, but which differs from the latter since it lacks C2a. In contrast, native C5 and C6 will produce EAC-6 only on reaction with ĒĀC̄4̄b̄,2̄ā,3̄b̄. Thus, cell-bound C2a is required for the reaction with native C5 but is not needed when C5 is supplied in the activated and complexed state C5b,6. Further, anti-C3 blocks the reaction of EAC4b,3b with C5b,6. These results suggest that in the reaction with native C5 the cell-bound C2a mediates the enzymatic activation of C5, whereas the cell-bound C3b supplies a binding site. In addition, the present experiments show that the activated state of C5b is stabilized by complexing with C6.