Immunotoxins are hybrid molecules composed of a monoclonal antibody chemically linked to a biologic toxin. The antibody portion of the molecule directs it to a specific antigenic determinant on a target cell; the molecule is then internalized, and a cytotoxic reaction occurs. First-generation immunotoxins were made by chemically coupling toxins such as ricin (a plant toxin) or diphtheria toxin to MAbs. The early products were highly immunogenic and chemically unstable, sometimes falling apart before reaching the tumor target. These difficulties led to the development of recombinant immunotoxins that were more stably linked to the antibody until bound on the target cell surface. Key words: Immunotoxin, Pseudomonous exotoxin, Monoclonal antibody