An "immunogenic" ribonucleic acid (Im-RNA) has been extracted from peritoneal exudate (PE) cells of rats that were immunized with sheep erythrocytes (SRBC). Following multiple phenol extractions and deoxyribonuclease treatment, the material obtained from PE cells was eluted from diethylaminoethyl-cellulose at 0.55 M NaCl concentration and partially purified in this procedure by a factor of 7- to 10-fold. After column chromatography, Im-RNA was found to be free of antigen based on results using (51)Cr-labeled SRBC or (14)C-dinitrophenol coupled to methylated bovine serum albumin as antigens. The Im-RNA showed a biphasic hyperchromicity curve when heated. The first phase, from 30 C to 90 C was gradual, accounting for 15.2% hyperchromicity suggestive of transfer RNA melting. No loss in immunogenic activity was observed when the Im-RNA was heated to 90 C. The second phase, from 90 C to 102 C, accounting for 15.2% further hyperchromicity, had a calculated T(m) of 96 C. Heating above 90 C resulted in an irreversible loss of immunogenic activity. These results strongly suggested that the RNA fraction contained a highly ordered secondary structure such as might be found with double-stranded nucleic acid. The nature and function of the Im-RNA is discussed.