1. As found by Sordelli and Mayer, agar adsorbed on bacteria produces agar-reacting antibodies in animals injected with these organisms.
2. False cross-agglutination and precipitin reactions can be produced in sera containing agar-reacting antibodies by organisms grown on agar. Zozaya's publication in this field (3) requires partial revision on this account.
3. There is suggestive immunological evidence of chemical similarity between the specifically reactive groups of agar-agar, and the C substance of the pneumococcus.