Summary Semi-synthetic medium for toxin production by C. diphtheriae is quite convenient in preparation and also economical. Toxin produced in the medium described above is as potent as that produced in the ordinary-infusion bouillon. Anatoxin prepared from this toxin does not flocculate when mixed with graded amounts of antitoxin and placed at 45°C. for a long period. This is only due to the presence of some inhibitory substance or substances in it which may be removed or modified by purification, dialysis or acidification. Antigenicity of this anatoxin was also proved directly on guinea pigs and horses. It compares very favorably with routine infusion anatoxin for the immunization of horses in the production of diphtheria-antitoxin.