Two pea seedborne mosaic potyvirus (PSbMV) isolates, P-1 DPD1 (P-1), which is highly seed-transmitted, and P-4 NY (P-4), which is rarely seed-transmitted, and chimeras between P-1 and P-4 were analysed to map the viral genetic determinants of seed transmission. Infectivity of chimeric viruses was evaluated by inoculating Pisum sativum with RNA transcribed in vitro from recombinant full-length cDNA clones. The chimeric viruses that were used demonstrated that a genomic segment encoding the 49 kDa protease and putative RNA polymerase was responsible for symptom induction. Attempts to determine transmission of the chimeric viruses in P. sativum cultivars known to transmit P-1 at high frequencies showed that seed transmission is a quantitative character influenced by multiple viral determinants. Seed transmission frequency did not correlate with accumulation of virus in vegetative tissue. The 5′ 2·5 kb of the 10 kb PSbMV genome had a major influence on the seed transmission frequency and was analysed further. This showed that, while the helper-component protease was a major determinant of seed transmission, the potyviral P1-protease exerted no measurable influence.