The group of dietary factors, the vitamin B complex, is included in the larger class of water-soluble vitamins because, like all vitamins, they are required by the body in small amounts and because the individual chemical compounds are soluble in water although the degree of solubility varies from the sparingly soluble riboflavin to the readily soluble choline. These are the main characteristics common to the individual vitamins because their functions in the living cell may differ greatly and their chemical structure may vary from the simple configuration of nicotinic acid to the more complex molecular structure of thiamine and riboflavin. The vitamin B complex is usually differentiated from the other water-soluble vitamins, such as vitamin C and related factors, on the basis of the source material used in early isolation work and the kind of experimental animals employed for the assay. No attempt will be made to cover in this