A wide range of toxins with interesting pharmacological properties have been isolated from ascidians. These include cyclic peptides, one of which, didemnin B, is undergoing clinical trials for cancer chemotherapy and is also a powerful immunosuppressive agent. Other classes of toxins from these organisms are alkaloids, macrolides, polyethers, prenylated hydroquinones and others of diverse chemical structure. The majority of these compounds are cytotoxic and in some cases the mechanisms of action have been investigated. The toxins are most likely used for chemical defense but the question of how the ascidians themselves escape the toxic actions is also not resolved.