Rabbits were immunized against yeast RNA, human ribosomes, and mouse myeloma ribosomal protein. Analysis of these antisera by double diffusion in agar against a variety of ribosomes and ribosomal RNAs demonstrated antigenic cross reactivity between ribosomes and isolated rRNA of varied mammalian species including rRNA from the rabbit, but not between ribosomal protein of varied species. Greater interspecies antigenic similarity of RNAs than of proteins appears to be based on a more limited structure of polynucleotides. © 1970, SAGE Publications. All rights reserved.