The coat protein of particles of sweet potato feathery mottle potyvirus (SPFMV) extracted from Ipomoea spp. migrated in SDS-PAGE mainly as bands of M(r) 38,000 (38K), 36K, 32K, 30K. Trypsin treatment of the particles resulted in the appearance of only one 30K polypeptide. The inclusion of protease inhibitors in the extraction procedure did not alter the heterogeneity of SPFMV coat protein. A partially purified fraction of extracts from recovering, symptomless, but not from healthy leaves of I. nil had a proteolytic activity similar to that of trypsin. Amino acid sequencing showed that the trypsin-cleaved 30K polypeptide had some sequence homology with other potyvirus coat proteins. The site at which the Ipomea extract cleaved the protein was five amino acids nearer the N terminus than trypsin cleavage site.