A full-length cDNA clone (pMCM41) was constructed to contain the exact 5' end of MCMV behind a T7 RNA polymerase promoter and a Smal site at the 3' end. Uncapped RNA synthesized from pMCM41 has the exact 3' end of viral RNA (vRNA) but is missing the cap found on vRNA. This RNA was infectious in protoplasts from black Mexican sweet (BMS) maize (Zea mays) suspension cultures. Uncapped transcripts were also infectious when inoculated onto maize plants and produced an infection indistinguishable from vRNA-inoculated plants. Capped pMCM41 transcripts which initiated at position +2 of the cDNA clone, as well as capped or uncapped RNA synthesized from a clone containing an extra G between the T7 promoter and the 5' end of MCMV sequence (pMCM721), were less infectious than uncapped pMCM41 transcripts in BMS protoplasts. The transcripts one nucleotide longer or shorter than uncapped pMCM41 transcripts were not able to infect maize plants.