Abstract Rana catesbeiana tadpoles formed high and low m. w. antibodies in response to immunization with a bacteriophage. Although the neutralizing activity associated with the low m. w. immunoglobulins was relatively weak, the existence of antibodies in this class was clearly demonstrated by radioimmunoelectrophoresis. Moreover, two antigenically distinct variants of the low m. w. antibodies were detected. These were serologically indistinguishable from the two types of low m. w. immunoglobulin previously isolated from the serum of adult frogs of this species.