Transgenic tobacco plants expressing the coat protein (CP) gene of the U1 strain of tobacco mosaic virus (TMV) exhibit CP-mediated resistance (CP-MR) against some, but not all, tobamoviruses. To investigate the role of the amino acid sequences on the surface of the challenge virus in CP-MR, mutant strains of U1 TMV were constructed to contain the amino or carboxy termini of the CP of Sunn hemp mosaic tobamovirus (SHMV). The modified virus was unable to overcome CP-MR in transgenic plants that contained the TMV CP. In contrast, TMV in which the CP was replaced by the SHMV CP overcame CP-MR to the same extent as did SHMV. We conclude that CP-MR conferred by TMV CP involves interactions between amino acid sequences of the challenge viruses and the transgene protein other than those on the surface of the challenge virus.Coat protein-mediated resistance (CP-MR) has been widely used to develop protection in monocots and dicots against a number of different viruses (see reviews by Fitchen & Beachy, 1993;Wilson, 1993). However, the molecular and cellular nature of the resistance has not been fully elucidated for any example of CP-MR. We have focused our studies of CP-MR on protection against tobacco mosaic tobamovirus (TMV) in tomato and tobacco plants. CP-MR conferred by the CP of TMV was highly effective against TMV and closely related tobamoviruses and less effective against the more distantly related Sunn hemp mosaic virus (SHMV; Anderson et al., 1989). The amino acid sequence of the CP of SHMV differs by about 60% from the CP sequence of TMV. It was proposed that the low level of amino acid sequence identity between the CPs of TMV strain U1 and SHMV contributes to the ability of the