1. 1. The oyster Crassostrea virginica was found to clear secondary injections of T2 bacteriophage more rapidly than primary injections. 2. 2. The clearance rates, however, were not as pronounced as in the lemon shark, Negaparion brevirostris. 3. 3. The response in the oyster was felt to be cellularly mediated since longterm immunization with T2 or typhoid vaccine failed to elicit detectable circulating antibody. 4. 4. Oysters injected with sheep erythrocytes in an attempt to raise the natural hemagglutinin titer also failed to respond in a fashion suggestive of an immune phenomenon. 5. 5. The physical-chemical properties of this natural hemagglutinin were determined and comparison made with the immunoglobulins of vertebrates. © 1969.