Ribonuclease F_1, a guanine specific endoribonuclease of Fusarium moniliforme, has been found to degrade rapidly poly (I). Other homopolyribonucleotides, poly (A), poly(C) and poly(U), are also degraded by the enzyme, though much more slowly than poly (I). The minimal substrates corresponding to these polymers, diribonucleoside monophosphates, are also degraded by the enzyme very slowly. These results show that the specificity of ribonuclease F_1 toward guanine base is strict hut not absolute.