Abstract. Using monoclonal antibodies directed against Xenopus immunoglobulins (Ig), the larval immune response was investigated and compared to that of the adult in some clones of isogenic Xenopus hybrids. The isotypes and the molecular nature of the adult and larval immunoglobulins were identical. IgM was first detected in the peritoneal fluid of larvae at developmental stage 49 (about 12 days), and low molecular weight (LMW) Ig was first detected in larvae at stage 50 (about 15 days). The ability of larvae to produce antibody increased during their development from stage 50–59 (about 15–45 days); the concomitant injection of primed, irradiated adult cells in addition to antigen increased the amount of antibody made, especially LMW Ig. The anti-DNP antibodies produced by larvae were of lower affinity than those produced by adults; sibling larvae given additional primed adult cells produced more antibody, but this was of larval-type affinity.