The immunological properties of a naturally-occurring double-stranded ribonucleic acid (ds-RNA), obtained from a mycophage of Penicillium chrysogenum, have been studied in relation to molecular size. Materials of reduced size, as reflected by molecular weight measurements, produced by ultrasonication of native ds-RNA, exhibited progressively lowered ability to induce an anti-ds-RNA response in mice. Adjuvant and immunosuppressive activities were of similar magnitude in both high and low molecular weight fractions. Evidence was also obtained of increased toxicity in materials of reduced size.