This invention relates to compositions and methods useful for activating LT-β receptor signaling, which in turn elicits potent anti-proliferative effects on tumor cells. More particularly, this invention relates to lymphotoxin heteromeric complexes formed between lymphotoxin-a and multiple subunits of lymphotoxin-β, which induce cytotoxic effects on tumor cells in the presence of lymphotoxin-β receptor activating agents. Also within the scope of this invention are antibodies directed against the lymphotoxin-β receptor which act as lymphotoxin-β receptor activating agents alone or in combination with other lymphotoxin-β receptor activating agents either in the presence or absence of lymphotoxin-a/β complexes. A screening method for selecting such antibodies is provided. This invention also relates to compositions and methods using cross-linked anti-lymphotoxin-β receptor antibodies either alone or in the presence of other lymphotoxin-β receptor activating agents to potentiate tumor cell cytotoxicity.