The paper reviews the information available concerning the mutagenic, carcinogenic and teratogenic effects of thallium. Data on mutagenic and carcinogenic risks of thallium and its compounds are extremely scanty but what is available does not indicate that thallium could be mutagenic or carcinogenic. At any rate, such risks, if they exist, would be submerged by the general high toxicity of thallium. On the other hand, thallium has some teratogenic properties, especially on cartilage and bone formation, although this seems to be more prominent in chicks than in mammals. Nevertheless, pregnant women should not be exposed to doses of thallium which might produce toxic symptoms; exposure to lower doses such as might occur near thallium-emitting plants is probably not embryotoxic.