The immunological response of the primitive mammal Tachyglossus aculeatus has been studied. We have shown that this animal responds to flagella from Salmonella adelaide with production of mercaptoethanol-resistant antibodies during both primary and secondary responses. Although an anamnestic secondary response could be demonstrated, it was of much smaller magnitude than that of phylogenetically higher placental mammals. Serological investigations demonstrated the presence of a mercaptoethanol-sensitive antibody of high molecular weight and an antibody of lower molecular weight which was mercaptoethanol resistant. These two antibodies seem to be analogous to IgM and IgG antibodies of higher mammals.