The necessity of coat protein for infection of plants by alfalfa mosaic virus (AIMV) and other ilarviruses distinguishes this virus group from other plant virus groups. Recently, the presence of both a zinc-finger type motif and zinc in AIMV coat protein was described [(1989) Virology 168, 48-56]. We studied the effect of a zinc chelator on viral RNA synthesis. Strong inhibition of AIMV RNA-dependent RNA polymerase (RdRp) by ortho-phenanthroline (OP) was observed.