A number of bacterial toxins act on targets located in the cytosol. Diphtheria toxin, Pseudomonas aeruginosa exotoxin A and shigella toxin inhibit protein synthesis by enzymatic inactivation of elongation factor 2 or the 60 S ribosomal subunit. These toxins enter the cells by receptor-mediated endocytosis, followed by translocation across the membranes of intracellular organelles. Also a number or toxins that are not cytocidal act on targets in the cytosol. A number of nontoxic bacterial proteins are able to modify enzymatically intracellular molecules. Some of these proteins could be considered for targeting to special cells followed by translocation to obtain defined physiological effects.