An RNase inhibitor has been isolated from the cytosol of fowl plague-infected chick embryo cells, which inhibits an RNase obtained from chicken serum. This inhibitor is a protein of rather low molecular weight. It does not bind to RNA. The best inhibition is obtained around pH 7. Its pH optimum is much sharper than that of the serum RNase. At relatively low concentrations of the inhibitor therefore the RNase exhibits a biphasic pH optimum. This RNase inhibitor is a useful tool for the study of the influenza RNA polymerase which is strongly inhibited by other RNase inhibitors like polyanions.