RNA-dependent RNA polymerase (RdRp) was solubilized and purified from cellular membranes isolated from alfalfa mosaic virus (AIMV)-infected tobacco by employing a procedure recently described for brome mosaic virus RdRp [R. Quadt and E.M.J. Jaspars, 1990, Virology 178, 189-194]. The purified AIMV RdRp is completely dependent on added template RNAs and exhibits a high degree of template specificity. Analysis of the protein composition of AIMV RdRp showed that AIMV-encoded proteins P1 and P2 and the coat protein (CP) are present in the active enzyme complex. Minus-strand synthesis by the AIMV RdRp is inhibited by AIMV CP. Native double-stranded AIMV RNAs are utilized as template for viral RNA synthesis by AIMV RdRp indicating that a helicase activity is present in the purified AIMV RdRp preparation.