The complement cascade is regulated to prevent inappropriate activation. This regulation is targeted not only at the initiation of the cascade but also at the amplification andjunctional (C3) steps. Five glycoproteins with both complement regulatory activity and binding affinity for C3b/C4b have been characterized. In plasma these molecules are factor H(H) and C4-binding protein (C4-bp) and, on cells, they are the C3b/C4b receptor (CR1), decay-accelerating factor (DAF), and gp45-70. Here Michael Holers and his colleagues review structural, functional and genetic studies of these proteins and discuss the evidence for a new multi-gene family with a common ancestral protein.