The naturally occurring secondary alcohols, known as the sterols, are classified biologically as zoosterols, phytosterols, and mycosterols, according to the source, animal, vegetable, or fungoid, from which they are obtained. The discovery that irradiation with short wave-length ultra-violet light confers powerful anti-rachitic properties on ergosterol, the principal member of the mycosterol sub-division, has given a great. impetus to the study of these compounds and of their derivatives. Mycosterols are mainly derived from the non-saponifiable matter in the fats of yeast and of ergot of rye, both of which contain complex mixtures of the individuals of this group. The separation of these mixtures has frequently been, attempted. and the isolation of many new sterols has been claimed, but the views of the various investigators are in such conflict that an independent and complete examination has become necessary.