Santesson made researches upon the poisonous action of benzene in connection with occurrence of certain cases of poisoning through ‘impure benzol’ (coal-tar benzene) in a rubber tyre factory. In the factory mentioned nine young women were poisoned, of whom four died. The symptoms shown were lassitude, anæmia, giddiness, headache, vomiting, and fever. Post mortem, hæmorrhages and fatty degeneration of the endothelium of the bloodvessels and various organs were found. Experimental research showed that commercial benzol and chemically pure benzene had the same effect. Santesson did not succeed in his experiments on animals in producing chronic poisoning by inhalation of benzine and of benzene fumes (which two completely different poisons he does not distinguish strictly from each other, as is the case, unfortunately, with many other writers). My experimental researches upon the poisonous effect of pure benzene, pure toluene, cumene, thiophene, and the most important kinds of commercial benzol gave the following results: For rabbits the limit of toxicity is a proportion of 0·015 to 0·016 per thousand pure benzene in the air, that is 0·015 to 0·016 c.c. benzene vapour per litre of air. A concentration of 0·056-0·057 per thousand pure benzene in the air causes in rabbits at once—after one minute—twitching of the muscles; after eight minutes, convulsions; after ten minutes, deep narcosis; and after twenty-five minutes, coma. If the animal is taken out of the bell in time, even if it has shown marked symptoms, it recovers very quickly (in two to ten minutes) without manifesting any after effects. Even in animals repeatedly exposed to the poison sequelæ were not observed. Dogs are somewhat more susceptible to pure benzene than rabbits; 0·024 per thousand causes after ten minutes severe convulsions, which after twenty minutes become continuous; 0·042 per thousand kills after twenty minutes (sudden death in a state of tetanus). Cats are less sensitive than dogs and more sensitive than rabbits; 0·03-0·04 per thousand causes after ten minutes attacks of cramp and, after twenty minutes, convulsions; 0·05 per thousand at once brings on poisoning symptoms. As regards the character of the symptoms (cramps, convulsions, quick recovery, no after effects) the above statements apply to all three kinds of animals (rabbit, dog, and cat). Chloral hydrate completely checks the convulsions and enables animals to tolerate higher concentrations of benzene for a longer time. Benzene is thus to be counted among nerve irritant poisons. The convulsions are probably provoked by excitement of the motor centres in the brain. In view of the fact that thiophene in a concentration of 0·03-0·05 per thousand in the air was borne by animals for an hour without producing any symptoms of poisoning, the proportion of thiophene in commercial benzol must be looked upon as practically non-injurious.