Rabbits were immunized with complexes of polyA-polyU and methylated bovine serum albumin, and the resulting sera were examined in two dimensional immunodiffusion and quantitative complement fixation. The sera reacted with the inducing copolymer, with polyl-polyC, with the double strand RNA of reovirus, and to a lesser extent with DNA-RNA hybrids. There was no reaction with polyA alone, polyU alone, polyG-polyC, or with a variety of non-duplex RNA or native or denatured DNA samples. Reactivity of double strand polyribonucleotides was abolished by effective thermal denaturation. © 1969.