An immunologic approach besides surgical, radiotherapeutic, chemotherapeutic methods in the treatment of cancer was proposed for the first time by Paul Ehrlich in 1906. The structure, produced by conjugating the antibodies against the tumor cells with a toxin molecule by Moolten and Cooperband (1970) was named as Immunotoxins. Producing antibodies with the desired amount and purity was achieved by improving the Hybridoma technology, Immunotoxins were tried to be produced by conjugating the monoclonal antibodies with the different toxic agents. Removing the receptor binding region of the toxin molecule was demonstrated to be more effective although good results were obtained from the studies utilising the toxin of Corynebacterium diphtheriae. Toxin production with the desired amount and structure was accomplished by the improved genetic methods. Although in vivo applications of the immunotoxins are still limited, it is believed that they will be important therapeutic agents in the future.