The relative incorporation of leucine- 3H or - 14C into proteins of virus-infected and healthy leaf tissue was used to detect virus mediated or virus-stimulated protein synthesis. Proteins from subcellular fractions of tobacco mosaic virus (TMV)-infected and potato spindle tuber virus (PSTV)-infected tissue were analyzed by electrophoresis in sodium dodecyl sulfate containing polyacrylamide gels. With TMV infections in tobacco, incorporation of leucine was stimulated into 4 proteins of molecular weights of 245, 195, 155, and 37 × 10 3 daltons (d) and into the coat protein monomer of 17.5 × 10 3 d. The predominant new or stimulated protein, found in all subcellular fractions was the 155 × 10 3 d component; the “nuclear” and “mitochondrial” fractions showed considerable stimulation into all 5 proteins. The “mitochondrial” fraction prepared from leaves infected with the defective TMV strain PM2I also showed stimulated incorporation into both the 155 × 10 3 d component and the nonfunctional coat protein. In extracts of PSTV-infected tomato plants no proteins of a molecular weight expected for a virus coat protein were detected, lending support to the conclusion that no protein-containing virions are produced by this unusual virus. However, incorporation of leucine was stimulated in two proteins which were approximately the size of the 155 and 195 × 10 3 d TMV-induced proteins.