Summary: The tuatara, Sphenodon punctatum, has been shown to produce antibodies to Salmonella adelaide flagellin. The serum of immunized animals contained immunoglobulins which possessed sedimentation coefficients of approx. 18S and 7S. The 18S immunoglobulins resembled the γM immunoglobulins of other vertebrates in size and polypeptide chain structure. The 7S immuneglobulins were antigenically related to the 18S proteins and possessed light chains resembling those of the latter on gel electrophoresis in urea. The heavy chains of the 7S protein differed from those of the macroglobulin, thereby indicating the presence of distinct immunoglobulin classes. Although the immunization period lasted for over eight months, antibody activity was found only in the 18S immunoglobulin.The lymphoid system of the tuatara was found to be primitive. The only clearly recognizable lymphoid organ in the adult animal was the spleen which showed a definite demarcation into white pulp and red pulp. Injected carbon particles were taken up only by the liver. The foreign particles in this organ were associated with phagocytic cells which resembled the Kupffer‐cells of the mammalian liver.The fact that this ancient reptile possessed multiple classes of immunogtobulins is consistent with previous findings that distinct immunoglobulin classes, defined by the presence of different heavy chains, were present by the phylogenetic level of higher amphibians.