An immunoconjugate consisting of the recombinant form of the cytokine interleukin-2 (IL-2) fused to a human single-chain Fv (scFv) antibody fragment directed against the extra-domain B (ED-B) of fibronectin (L19), with potential immunopotentiating and antineoplastic activities. The L19 moiety of L19-IL2 monoclonal antibody-cytokine fusion protein binds to the ED-B domain of fibronectin on tumor cells in the tumor neovasculature. In turn, the IL-2 moiety may locally activate natural killer (NK) cells and macrophages, and may induce T cell cytotoxic immune responses against ED-B fibronectin-expressing tumor cells. This may specifically decrease the proliferation of ED-B-expressing tumor cells. ED-B is predominantly expressed during angiogenesis and tumor growth.