It will be evident that in the absence of exact tests for a substance which is unknown chemically the problem of detecting its presence must be a matter of indirect evidence. When a chemist is presented with a solution and asked to determine the presence or absence of lead in that solution he knows what he is seeking, what its properties are and how to proceed to not only determine its presence but to measure exactly the amount present. No such possibility is present in a test for vitamines, but this lack of knowledge as to the vitamine structure has not left us helpless. We do know enough of its action to permit us to detect its presence and the technique that has been developed for this purpose is now well standardized and involves no mysteries beyond the comprehension of the layman. In the present chapter is outlined the development of vitamine testing together with a discussion of some of the deficiencies and the problems for the future that these deficiencies suggest. When Casimir Funk made his original studies of the chemical fractions of an alcohol extract of rice polishings he utilized a discovery of the Dutch chemist Eijkman. We have already referred to this discovery, viz., that by feeding polished rice to fowls or pigeons they could be made to develop a polyneuritis which is identical in symptoms and in response to the curative action of vitamine, to the beri-beri disease. A normal pigeon can be made to eat enough rice normally to develop the disease in about three weeks. The interval can be somewhat shortened by forced feeding. As soon as the symptoms develop the bird is ready to serve as a test for the presence or absence of the antineuritic vitamine. If at this time we have an unknown substance to test it can be administered by pushing down the throat or mixed with the food or an extract can be made and administered intravenously. If the dose is curative, the bird will show the effect by prompt recovery from all the symptoms of the disease in as short a time as six to eight hours. Such a procedure provides a qualitative test which can be made roughly quantitative by varying the dosage until an amount, just necessary to cure the bird in a given time is found and then expressing the vitamine content of the food in terms of this dosage, in such an experiment the value is obviously based on the curative powers of the vitamine source. Another way of applying the test is to determine just how much of the unknown must be added to a diet of polished rice to prevent the onset of polyneuritic symptoms. Such a determination will give the content in terms of preventive dosage. Both methods have been extensively applied and the following tables compiled from the Report of the British Medical Research Committee illustrate both the method and some of its results: