Following immunization with Salmonella typhosa H (STH) antigen, the marine toad was found to synthesize STH agglutinating antibody associated with the 17.9S fraction of sera. When the animals were challenged with T2 bacteriophage, T2 neutralizing antibody was found in both 17.9S and 7.3S fractions of sera at 21 days after primary immunization. At 14 days after a secondary injection of T2, no shift to the 7.3S fraction was observed. These immunoglobulins, designated immune macroglobulin and low molecular weight antibody, were purified and found to have molecular weights of ~880,000 and ~160,000, respectively. The immunoglobulins were also found to differ from each other in carbohydrate composition. Although the L chains from both molecules had a molecular weight of ~22,500, the H chain for the immune macroglobulin was ~67,000, while that of the low molecular weight antibody was ~53,000. In addition, the H chains of both classes of immunoglobulins were distinguishable from each other on the basis of amino acid compositions, peptide maps, and NH2-terminal amino acid sequences. Thus, evidence is presented which clearly demonstrates the presence of at least two distinct classes of immunoglobulins in the marine toad. © 1972, American Chemical Society. All rights reserved.