A fusion polypeptide comprising (A).sub.x-M-(A').sub.y, wherein A and A' are each polypeptides capable of binding a target receptor. The fusion polypeptides of the invention form multimeric proteins which activate the target receptor. A and A' may be each be an antibody or fragment derived from an antibody specific for a target receptor, such as the same or different ScFv fragments, and/or a ligand or ligand fragment or derivative capable of binding the target protein, M is a multimerizing component, and X and Y are independently a number between 1-10.