The mechanism of Pisum sativum pathotype-specific resistance to pea seed-borne mosaic potyvirus (PSbMV) was investigated and the coding region determinant of PSbMV virulence was defined. Homozygous recessive sbm-1 peas are unable to support replication of PSbMV pathotype 1 (P-1), whereas biochemically and serologically related pathotype 4 (P-4) is fully infectious in the sbm-1/sbm-1 genotype. We were unable to detect viral coat protein or RNA with double antibody sandwich-enzyme-linked immunosorbent assay and reverse transcription-polymerase chain reaction in sbm-1/sbm-1 P-1-inoculated protoplasts and plants. Lack of viral coat protein or RNA in P-1 transfected sbm-1/sbm-1 protoplasts suggests that sbm-1 resistance is occurring at the cellular level and that inhibition of cell-to-cell virus movement is not the operating form of resistance. In addition, because virus products were not detected at any time post-inoculation, resistance must either be constitutive or expressed very early in the virus infection process. P-1-resistant peas challenged with full-length, infectious P-1/P-4 recombinant clones demonstrated that a specific P-4 coding region, the 21-kDa, genome-linked protein (VPg), was capable of overcoming sbm-1 resistance, whereas clones containing the P-1 VPg coding region were noninfectious to sbm-1/sbm-1 peas. VPg is believed to be involved in potyvirus replication and its identification as the PSbMV determinant of infectivity in sbm-1/sbm-1 peas is consistent with disruption of an early P-1 replication event.