An immunocytokine consisting of human pro-inflammatory cytokine tumor necrosis factor alpha (TNFalpha) fused to a human single-chain variable fragment (scFv) directed against the extra-domain B (ED-B) of fibronectin (L19), with potential immunopotentiating and antineoplastic activities. Upon adinistration, the L19 moiety of onfekafusp alfa binds to the ED-B domain of fibronectin on tumor cells in the tumor neovasculature. In turn, the TNFalpha moiety may locally induce an immune response against ED-B fibronectin-expressing tumor cells and may specifically decrease the proliferation of ED-B-expressing tumor cells. ED-B is predominantly expressed during angiogenesis and tumor growth.