Nicotiana tabacum cv. Burley 49 germplasm, transformed with mutated versions of the tobacco etch virus (TEV) coat protein gene, was tested under field conditions for tolerance and resistance to TEV. Twenty-four independent transgenic lines and eight different mutated versions of the TEV coat protein gene were examined. The test was conducted at the same field location for two successive years. Transgenic plants expressing an mRNA, which was translated into a full length TEV coat protein, became infected when challenged with TEV but then recovered from TEV infection 3-4 weeks later. The same response was displayed by plants containing a gene lacking the codons for 29 amino acids at the amino terminus of the TEV coat protein. Plants containing a gene that expressed an antisense RNA version of the TEV coat protein sequence displayed little tolerance to TEV. A similar response to TEV was observed with plants containing only regulatory gene sequences and no TEV-derived sequences, or with plants having no foreign gene sequences at all (Burley 49). Selected entries expressing untranslatable versions of the TEV coat protein mRNA never became infected with TEV, and they displayed a high level of resistance. This study confirms the use of transgenic plants expressing an untranslatable sense RNA as a promising approach in the development of Potyvirus-resistant tobacco germplasm.