In many plant RNA viruses, Domains 1, 2 and 3 are conserved in replicase proteins. In order to examine the interference of viral replication by the Domain 1 sequence, we generated transgenic plants transformed with DNA corresponding to the Domain 1 sequence of the TMV 126 kDa protein. This DNA sequence includes the TMV RNA from nucleotides 1 to 2,149, which comprises both the 5'-untranslated and methyl transferase region. The transgenic plants obtained showed complete resistance to TMV infection. The presence of the Domain 1 sequence in the plants completely prevented local necrosis in Nicotiana tabacum cv. Xanthi nc, and any systemic development of symptoms in Nicotiana tabacum Xanthi upon TMV inoculation. Most transgenic plants sustained the conferred resistance even under TMV inoculum concentrations up to as high as 1,000 microg/ml. To detect any accumulation of TMV coat protein or viral RNA in infected transgenic plants, immunochemical tests and Northern blot analyses were carried out. Neither viral RNA or coat protein was detectable in the systemic leaves of the completely resistant transgenic plants, whereas they were accumulated in large quantities in all of the control plants. Because of the conservation of Domain 1 in many plant RNA viruses, the acquisition of resistance to virus infection using the Domain 1 sequence appears to be a very effective strategy for breeding of viral resistant plants.