Abstract The antibody-synthesizing capacity of tadpoles (Rana catesbeiana) has been studied with sheep erythrocytes as antigen. Tadpoles can mount an immune response similar to that seen in higher vertebrates except for a slower kinetic profile. They could not, however, be stimulated to elicit a true anamnestic response. The antibody had characteristics indicating that it probably was all IgM like. Use of the hemolytic plaque technique with various lymphoid organs detected synthesis of antibody in spleen, lymph gland and thymus.