Complement is not only part of the innate immune system, but has also been implicated in adaptive immunity. The role of complement and its regulatory proteins in modulating T cell activity has been the focus of several recent studies. These, which have included work on the membrane co-factor protein (MCP or CD46), decay accelerating factor (DAF or CD55) and CD59, indicate that complement regulators can influence the proliferative capacity of T cells and their ability to produce cytokines, influencing the outcome of a T cell response to a given antigen. Here we review these studies, which reveal another important link between the innate and the adaptive immune system.