The two most widely promoted vitamins are vitamin A and vitamin D. No satisfactory methods of physical or chemical assay have been devised for them. Animal assays of course are used and are satisfactory for most clinical practice; but they are costly, require a large expenditure of time, and their accuracy does not nearly approach that for ordinary analysis. Therefore attempts to standardize preparations by other than bio-assays are always welcome. In this direction the Medical Research Council of Great Britain has been fostering a series of reports on products now biologically standardized. The two hundred and second report of this council deals with the standardization and estimation of vitamin A.
After the introduction there is an excellent chapter dealing with carotene as the international standard of vitamin A. This alone is a complex subject in view of the fact that carotene exists in three isomeric forms—alpha, beta and gamma—and,