Inhibition of S-adenosylhomocysteine (AdoHcy) hydrolase by compounds such as Neplanocin A (NPA) leads to the build-up of AdoHcy and the inhibition of methyltransferase enzymes. Whether assayed by efficiency of plaquing or virus yield, SVLM21, a mutant of Sindbis virus resistant to methionine deprivation, was more sensitive to NPA than was the standard virus (SVstd) from which it was derived. For example, whereas 10 micrograms NPA/ml depressed the yield of SVLM21 by more than 30-fold, the yield of SVstd was not significantly affected. Similar differences in sensitivities were shown to three other compounds which inhibit AdoHcy hydrolase. These results support the idea that SVLM21 codes for an altered RNA methyltransferase.