Studies of decay-accelerating factor (DAF) function and structure are reviewed. DAF was first recognized as a species restricting factor operating at the level of C3/C5 activation. Cloning of the gene indicates that DAF has four short consensus repeats of the type characteristic of the regulators of complement activation gene cluster family. The third short consensus repeat is responsible for DAF's complement regulatory activity and signaling. DAF, like other glycophosphatidylinositol (GPI) anchored proteins, is associated with tyrosine kinases, and these kinases are probably the signaling devices. The details of how DAF's GPI anchor in the outer leaflet of plasma membrane connects with the tyrosine kinases on the inner leaflet are not known. Although DAF does not have an essential role in controlling hemolysis of erythrocytes, it does have important role in regulating the deposition of C3 on nucleated cells. The therapeutic potential of DAF is discussed.