1. Antibodies to slime molds were produced by injecting D. discoideum and D. purpureum amebas from 48 hour cultures into rabbits.
2. Anti-D. discoideum and anti-D. purpureum sera caused agglutination of homologous amebas from 24 to 26 hour cultures, agglutination of certain heterologous amebas from 30 to 36 hour cultures, and agglutination of all heterologous amebas from 43 to 48 hour cultures.
3. The data show that new surface antigens are formed in cultures after 26 hours and it is suggested that the new antigens are concerned with cell adhesion.
4. The probable role of surface antigens in the interaction of cells of different species of slime molds was discussed.