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Actinomycin D (AMD), 50, 10, and 1 μg/ml, stimulated potato virus X (PVX) multiplication in detached Datura stramonium L. leaves floated on water. Increased virus synthesis was observed upon treatment of the leaves with AMD at 2, 6, and 24 hr before, or at the time of, inoculation as well as 24 hr later. Treatment of leaves with AMD prior to inoculation proved most effective. The stimulating effect depended on both AMD concentration and time; it increased with concentration changes of from 1 to 50 μg/ml, as well as with lengthier treatment in AMD solutions up to 24 hr. Increased PVX multiplication in AMD-treated leaves was observed by bioassay of virus on Gomphrena globosa L. leaves, testing both the homogenates and a partially purified viral preparation obtained from inoculated D. stramonium leaves. The stimulated PVX multiplication in AMD-treated D. stramonium leaves was observed in the presence of suppressed cellular metabolism; this was testified to by data obtained with the aid of electronic paramagnetic resonance (EPR) measurements. Possibly, increased PVX multiplication in AMD-treated D. stramonium leaves occurs as a result of the DNA function being suppressed by the antibiotic causing an interruption in cellular syntheses, thus leading to the primary use of the cell energy resources and metabolic precursors in viral synthesis.