The particulate fraction of a homogenate of alfalfa mosaic virus infected tobacco was found to contain the viral replicase, and one or more host-specific RNA-polymerases. Using an endogeneous template, the membrane-bound replicase incorporates 3H-CTP in virion-type RNA. The synthesized RNA is part of a replicative intermediate; after RN-ase-treatment the product comigrates with viral double-stranded RNA in polyacrylamide gels. A preliminary characterization of this double-stranded RNA was made by RNA-RNA hybridization. Treatment of the particulate fraction with lubrol releases a host-specific RNA-polymerase. The activity of this enzyme is completely dependent on exogeneous template-RNA. Probably only a very small region of the template is transcribed. After washing with lubrol, the particulate fraction still contains the viral replicase. When this fraction is resuspended in a Mg++-deficient buffer, about 60% of the enzyme activity is released into the supernatant. No such activity is found in a comparable extract of healthy leaves.