Formation of from and C5 involves physical uptake of the C5 molecule or, more likely, the C5b fragment by non-covalent forces. The fixed C5b tends to dissociate at high ionic strength or elevated temperature by one, or both, of two mechanisms. The first of these involves dissociation of C5b as a consequence of decay-release of cell-bound C2 at elevated temperature. The second mechanism entails release of C5b independently of the decay-release of C2. for a very brief period of time following release, the C5b fragment retains its hemolytic activity in the sense that it can reassociate with the complex and function in the subsequent steps of the reaction sequence leading to hemolysis. However, if it does not have the opportunity to reassociate with , the C5b fragment quickly loses hemolytic activity. The fleeting existence of hemolytically active C5b in the fluid phase can be demonstrated by transfer experiments in which donor cells, , are placed in close proximity to recipient cells, , so that the recombination can take place before reactivity is lost.