Following immunization with human IgG three species of anuran amphibians, Xenopus borealis, Xenopus clivii and Xenopus muelleri, were found to synthesize two molecular populations of antibodies associated with 19S and 7S fractions of the sera. These antibodies, designated high (HMW) and low (LMW) molecular weight immunoglobulins, were isolated and their constituent heavy (H) and light (L) polypeptide chains separated following extensive reduction and alkylation in a relative yield of about 70% and 30% respectively. The molecular weights of H and L chains of the three species were determined by SDS-acrylamide gel electrophoresis. The L chains from both molecules in the three species had a molecular weight of about 26,000. The H chains of the HMW and LMW immunoglobulins had a molecular weight of about 73,000 and 63,000 respectively. The two populations of immunoglobulins were analysed in immunodiffusion plates using rabbit antisera to LMW immunoglobulins. The HMW immunoglobulins were found to be antigenically deficient with respect to LMW immunoglobulins. From these results it was judged that the two populations in each species belonged to distinct classes. The antigenicities of the LMW and HMW immunoglobulins of different species within the genus Xenopus were compared.