The invention relates to novel trimeric costimulatory TNF family ligand-containing antigen binding molecules comprising three fusion polypeptides, each of the three fusion polypeptides comprising (a) an ectodomain of a costimulatory TNF family ligand selected from the group consisting of 4-1BBL, OX40L and GITRL or fragments thereof, (b) a trimerization domain, in particular a trimerization domain derived from human cartilage matrix protein (huCMP) of SEQ ID NO:1, and (c) a moiety capable of specific binding to a target cell antigen, and to methods of producing these molecules and to methods of using the same.