We have modified PE.sub.40 toxin by removing at least two of its four cysteine amino acid residues and have formed hybrid molecules containing modified PE.sub.40 linked to a cell recognition protein that can be an antibody, a growth factor, a hormone, a lymphokine, or another polypeptide cell recognition protein for which a specific cellular receptor exists whereby the modified PE.sub.40 toxin is directed to cell types having receptors for the cell recognition protein linked to the modified PE.sub.40.