This chapter discusses the preparation of interferon messenger RNAs with the use of ribonucleoside-vanadyl complexes. Ribonucleoside-vanadyl complexes can be used in a wide variety of situations in which a nuclease inhibitor is required. These materials can protect RNA from degradation without affecting other cellular components because they are specifically aimed at ribonucleases. They have been used successfully to prepare polysomal interferon mRNA and to prepare and fractionate ribosomes and polysomes. When Namalva cytoplasm is incubated at 4° for 1 hr in the presence of the complexes, no destruction of interferon mRNA is observed. By utilizing ribonucleoside-vanadyl complexes it is no longer necessary to denature proteins or deproteinize solutions early in the isolation of mRNA. Since these substances are easy to prepare and handle, they should be considered whenever RNA extractions are anticipated. Any comparison of interferon mRNA yields should take into account the inherent errors in the assay methods. The two largest variables in the yield of interferon mRNA from leukocytes are the amount actually made by the cells and the time of the harvest. To purify interferon mRNA, polyadenylated molecules must be separated from all others. © 1981, Academic Press, Inc.