The gametophytic self-incompatibility system of Solarium tuberosum is controlled by a single locus, designated as the S-locus. Protein extracts from potato styles of defined S-genotypes have been analysed by two-dimensional gel electrophoresis, and found to contain a group of basic glycoproteins. Each genetically determined allele S 1 to S 4 was associated with the presence of one of a number of these polypeptides differing slightly in isoelectric points (in the range 8.3->9.1) and/or apparent molecular weight (ranging from 23,000 to 29,000). Two abundant basic polypeptides, one of which is apparently not glycosylated, were present in all genotypes examined. Amino-terminal protein sequence determinations revealed homologies of the S. tuberosum stylar proteins S2, S3 and S4 with SI-associated polypeptides from Nicotiana alata and Lycopersicon peruvianum. With an oligonucleotide generated to the potato-S2 N-terminal protein sequence, it was possible to detect a style-specific RNA species of ∼920 nucleotides. The oligonucleotide also behaved as an allele-specific probe when hybridized to total RNA of different S-genotypes.