T HE notion is not a new one that the thymus might have something to do with myasthenia gravis; this is obvious from publications on the subject since the beginning of this century. Furthermore, it is clear that interest in the experimental investigation of the subject has arisen and has been maintained as a result of consistent clinical observations on patients with myasthenia gravis. The idea of trying to produce a myasthenic animal or at any rate of attempting to reproduce the essential features of this condition in the whole or part of an animal has occurred to many, but the results of such attempts as have been published are conflicting and in general have provided little evidence that the thymus influences muscular function. I shall not comment on the results of implanting pieces of thymus gland other than to draw attention to the fact that the duration of effect was transient, and that in no case has the tissue been transplanted to ensure vascular anastomosis with the recipient. I would like briefly to summarise the published evidence regarding the effects of thymus extracts, for statements are often made about findings by one author not being confirmed by another, when in fact the experiments have not been repeated. In most instances calf thymus extracts have been used; a few extracts have also been made with thymus glands removed from patients with myasthenia gravis. The extracts of calf thymus have usually been made with saline solution and tested in doses equivalent to 0.4 to 20 gin. of fresh thymus. The biologic preparations used to assess the activity of the extracts have been of several types; the method has usually been to produce muscular weakness or fatigue in the unanaesthetised animal or in a nerve-muscle preparation by stimulation of the nerve supplying a muscle or group of muscles, and to observe whether the extract prevented, relieved or induced muscular fatigue. Adler produced in the dog rapid fatigue of the anterior tibialis muscle in response to repeated indirect tetanic stimulation, after intravenous injection of an extract equivalent to 4 gm. of fresh calf thymus. This effect lasted for about twelve hours and was relieved and prevented by neostigmine. When the extract was injected subcutaneously the effect was delayed in onset but lasted for about twenty-four hours. Extracts of calf spleen did not produce these effects. Klose and Vogt used pressed thymus juice in doses equivalent to 10 to 20 gin. of fresh thymus. When this was injected intravenously or subcutaneously into thymectomised puppies it aggravated the signs of fatigue already evident in these animals. Normal dogs were unaffected. Constant and his colleagues observed, in the cat, depression of the muscle twitch in response to stimulation of its nerve after a saline extract equivalent to 2 gm. of dog thymus was injected into the carotid artery. A number of attempts have been made to extract thymus glands from myasthenic patients and to test them for curare-like activity. The only evidence of a positive nature is that by Constant and his colleagues who observed depression of the muscle twitch after intracarotid injection of a saline extract equivalent to 1 to 2 gm. of fresh thymus gland. Choice of Solvent, Biologic Test and Standard. The negative results reported in many of these investigations are probably due to the use of relatively insensitive biological tests and to insufficient doses of thymus, for an estimate of the amounts of tubocurarine required to produce an effect on these test objects shows that each dose of thymus would require to contain curarelike activity equivalent to 0.1 to 1 mg. d-tubocurarine. This is beyond the usual total activity obtained from one myasthenic thymus gland. McEachern, at least, was aware of some of these difficulties when he briefly reported his results in a masterly review of the subject ten years ago. Our first extracts were prepared from myasthenic thymus glands and consisted of prelimi* From the University of Liverpool, Liverpool, England.