Abstract: Antitumor protein hybrid, having a moiety consisting of the antitumor immunoglobulin and a moiety substantially consisting of the fragment A of a diphtheria toxin, which is expressed by the following formula (I):Ab(NHCO--X--S.sub.1 --S.sub.2 --FA).sub.n (I)(where Ab indicates a moiety consisting of the antitumor immunoglobulin and FA indicates a moiety substantially consisting of the fragment A of a diphtheria toxin; X is an alkylene group or phenylene group which either has or does not have a branching of 1 to 5 carbon atoms. N in the amido bond is a nitrogen atom arising from the amino group in the antitumor immunoglobulin; S.sub.1 and S.sub.2 are both sulfur atoms, S.sub.2 indicating a sulfur atom arising from the disulfide bond in a diphtheria toxin; n stands for an integer of 1 to 5).This antitumor protein hybrid has remarkable and specific citotoxicity against tumor cells.

Abstract: Antitumor protein hybrid, composed of a moiety which is substantially the fragment Fab of an anti-.alpha.-fetoprotein antibody and a moiety which is substantially the fragment A of a diphtheria toxin, which is expressed by the following formula (I):Fab--(S.sub.1 --(X).sub.n --S.sub.2 --FA).sub.m (I)(where Fab indicates a moiety which is substantially the fragment Fab of an anti-.alpha.-fetoprotein antibody; FA indicates a moiety which is substantially the fragment A of diphtheria toxin; X indicates a divalent organic radical; S.sub.1 and S.sub.2 are both sulfur atoms, S.sub.1 indicating a sulfur atom arising from the disulfide bond (--S--S--bond) in an anti-.alpha.-fetoprotein antibody and S.sub.2 a sulfur atom arising from the disulfide bond in a diphtheria toxin; n stands for 0 or 1 and m stands for an integer of 1 to 5). This antitumor protein hybrid has remarkable and specific citotoxicity against tumor cells.