In the vast literature already existing about Ehrlich's wonderful discovery there is little experimental evidence on the action of salvarsan in acid solution when given intravenously. The drug has been occasionally given in this form to human beings by a few clinicians, but their results need not concern us here, for it is obvious that the best method for administering an admittedly toxic substance cannot be thoroughly tested on human beings. These considerations made it desirable to investigate this aspect of the subject, and therefore the tolerance of rabbits for salvarsan was tried out when this substance was dissolved in 0.9 per cent, salt solution and injected intravenously. Practically the only experimental contribution which demands consideration here has been the work of Hering,1 who tested the effect of 606 when dissolved in 0.9 per cent. saline so as to form a 0.5 per cent. solution.