The relationship of lipoids to immune reactions may be considered from a dual standpoint, first their possible antigenicity, and second their presence and possible function in immune sera. Only the latter of these is within the scope of this report; for the present study was undertaken to ascertain whether or not lipoid content is increased in serum of horses after they have been immunized against pneumococci. Earlier work of such a nature has been limited. In 1908 Takaki (1) reported the average weight of alcohol-chloroform extracts of various antitoxic sera (tetanus, diphtheria, and others) as 0.58 gram per 100 cc. serum as contrasted with 0.41 gram per 100 cc. of normal serum. His figures thus show a significant increase of lipoidal material in certain antitoxic sera. Koldaev (2) in 1921, however, found no difference in the amount of cholesterol in the serum of normal horses or of horses immunized against tetanus, diphtheria, or typhoid bacilli.