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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.

Three distinct immunoglobulins (Igs) have been isolated from serum of Rana catesbeiana frogs. One of these is high in molecular weight and probably corresponds to the IgM-like Igs that have been isolated from a variety of vertebrate species. The other 2 Igs are lower in molecular weight (approximately 7S) and very similar in subunit structure. They were highly cross-reactive, although each contained unique antigenic determinants. Their relationship to other vertebrate Ig classes remains to be established. The mode of subunit linkage in all 3 Igs is unusual since the unreduced proteins were partially dissociated in detergent. Serum from R. catesbeiana tadpoles contained Igs that were antigenically identical to each of the Igs in adults of this species. Sera from R. pipiens and R. clamitans frogs and tadpoles contained Igs that cross-reacted with the R. catesbeiana high and low molecular weight Igs.

Two varieties of low m.w. immunoglobulins have been isolated from the serum of Rana catesbeiana frogs. They are highly cross-reactive, although each also contains unique antigenic determinants. Since both low m.w. immunoglobulins were identified in the serum of 22 individual frogs, it was concluded that they are isotypic variants. The light chains of R. catesbeiana and mammalian high and low m.w. immunoglobulins are similar in electrophoretic mobility on polyacrylamide gels containing sodium dodecyl sulfate. The heavy chains of fropg high m.w. immunoglobulins have the mobility of mammalian mu-chains; the heavy chains of both variants of frog low m.w. immunoglobulins migrate between mammalian mu- and gamma-chains in approximately the position of mammalian alpha-chains. An unusual structural feature of the R. catesbeiana high ald low m.w. immunoglobulins is that the unreduced proteins are partially dissociated in sodium dodecyl sulfate.