The ribonucleic acid (RNA) synthesis of tobacco leaf tissue was inhibited by about 90% when the tissue was infiltrated with actinomycin D (AMD) at 50 μg/ml for 2 hours before labeling with uracil-14C without affecting pool size. Smaller doses given for longer periods, or higher doses for shorter periods, were less effective. Long exposures, even to small doses of the drug, reduced the uptake of uracil-14C by both healthy and tobacco mosaic virus (TMV)-infected tissue. Application of the optimal conditions of treatment for the suppression of cellular RNA synthesis to infected tissue enhanced the synthesis of TMV-RNA. Moreover, infected tissue exhibited an excess AMD-resistant RNA synthesis not accounted for by the RNA incorporated into virus particles. This RNA sedimented mainly in the position occupied by the light ribosomal RNA in sucrose density gradients. © 1969.