Previous work in our laboratory has centered on the immunoglobulins (Ig) and immune response of the Syrian hamster. Various other hamster species are becoming available (1), however, and will be useful as models in medical research. The question arises whether these hamsters have Ig’s comparable to and antigenically cross-reactive with those described in the Syrian hamster, so that antisera to previously characterized serum proteins of Syrian hamsters would be useful in studies with non-Syrian hamsters. In addition, the degree of identity of the various Ig classes in these other species would provide some idea of their relative taxonomic distance from Syrian hamsters. To answer these questions sera fromheterologous non-Syrian hamsters were tested on simple gel diffusion for precipitin reactions with rabbit antisera specific for Syrian hamster IgG1, IgG2, IgA, and IgM (2). The hamsters also were evaluated for the presence of an interesting sex-limited serum protein called Female Protein (FP) (3). In the diffusion analysis, the antisera were reacted with the heterologous hamster sera and an adjacent well contained homologous Syrian hamster sera so that the heterologous protein could be evaluated for its antigenic identity (+ = fusion reaction) or antigenic deficiency (+ = spur formation) with the homologous interaction.