A fusion protein of single-chain, Fv-tumor necrosis factor alpha, scFv-TNF, was constructed and expressed in the Pichia pastoris expression system. There was 67-fold less toxicity of the fusion protein when compared with TNF-alpha alone in cells expressing CA125. Furthermore, scFv-TNF was 10-fold more selective in cell killing directed by anti-CA125 scFv in a CA125-positive cell line than the CA125-negative line.