Abstract: The present invention provides a novel antibody-RNase conjugate which is a single chain protein, providing both the specificity of its antibody portion and the RNase activity of its RNase portion, resulting in an antigen specific effectiveness against cells when applied in vivo or in vitro, wherein the RNase portion is effectively cytotoxic in at least a fraction of cells presenting the antigen, e.g. after internalization by endocytosis. In detail, the present invention provides scAb-RNase conjugate having the principal structure of scFvFc-RNase. This structure could also be shown to allow the effective production of antigen-specific and cytotoxic conjugate protein in cell culture, the conjugate having a high activity with respect to antigen specificity and cyto toxicity.