ADP-ribosylating toxins from Neisseria meningitidis, Streptomyces coelicolor, Mycoplasma pneumoniae, Salmonella typhimurium, Salmonella paratyphi, and Streptococcus pyogenes are disclosed, together with mutant toxins and uses therefor. There is only a low level of sequence identity between these toxins and toxins such as cholera toxin and E. coli heat labile toxin.