When cell-free preparations containing large molecules and subcellular bodies extracted from tobacco-leaf cells infected with tobacco mosaic virus were incubated with MgCl(2) and the four ribonucleoside-5'-triphosphates, the level of infectivity increased about threefold in 30 minutes. The mechanism synthesizing the new infectivity was believed to be located within isolated nuclei. Ultrasonic rupture of the nuclear membranes appeared to increase the rate of synthesis.