Self-incompatibility in the grass Phalaris coerulescens is controlled by two genes S and Z. Isolation and sequencing of two S alleles showed that they encode proteins that are highly conserved at the C terminus with significant homology to thioredoxin H proteins. In particular, the residues in and around the active site of thioredoxin, Trp-Cys-Gly-Pro-Cys, are perfectly conserved. The C terminus of the S protein has been expressed in Eschericia coli and purified to homogeneity on Ni-NTA resin. Functional assays showed that the protein has thioredoxin activity; it can act as a substrate for E. coli thioredoxin reductase and also catalyse the reduction of insulin by dithiothreitol. The possible role of thioredoxin-like activity of the S protein in mediating the incompatibility reaction in Phalaris is discussed.