The Efb protein from Staphylococcus aureus has now been shown to have the ability to bind to the C3 protein which is a crucial component in the activation of complement, and a specific C3 binding region has been located at the C-terminal end of the Efb protein. Isolated proteins and protein fragments containing the Efb protein C3 binding region are thus provided which have complement inhibiting activity, and these proteins and fragments are particularly useful in therapeutic methods wherein the inhibition of complement is desirable, such as in the treatment of hemolytic anemia, the prevention of graft or implant rejection, and to alleviate complement activation that is associated with kidney dialysis methods such as hemodialysis.