1. The addition of cholesterin to an alcoholic extract of heart or fetal liver increases the antigenic value of the extracts in the Wassermann reaction.
2. The optimum amount of cholesterin to be added to heart extract or fetal liver extract was found to be 0.4 per cent.
3. Cholesterin-heart extracts are superior to cholesterin-liver extracts and to alcoholic extracts of syphilitic livers, as well as to ether extracts of dried hearts.
4. Cholesterin-heart extracts prepared from different human hearts are practically equal in anticomplementary and antigenic value. Similar extracts prepared from guinea pig hearts have the same antigenic value as those prepared from human hearts. Both the human heart and the guinea pig heart extracts are superior to beef heart extract when the same amount of cholesterin is added to each of the extracts.
5. In testing blood serum for diagnostic purposes, it is not safe to use more than one fourth of the anticomplementary dose of the 0.4 per cent. cholesterin heart extract. In the work here presented, this consisted of a 1 in 10 emulsion.
6. In testing cerebrospinal fluids, 1 in 10 emulsions give slightly better reactions with smaller quantities of the fluid than do 1 in 6 emulsions.
7. Because of the simple preparation, the superior antigenic property, and the constant antigen value of cholesterin-heart extracts prepared from human hearts, we agree with McIntosh and Fildes that this form of extract fulfills the requirements of a standard antigen.