A detailed analysis of the currently available Arabidopsis thaliana genomic sequence has revealed the presence of a large number of open reading frames with homology to the stigmatic self-incompatibility (S) genes of Papaver rhoeas. The products of these potential genes are all predicted to be relatively small, basic, secreted proteins with similar predicted secondary structures. We have named these potential genes SPH (S-protein homologues). Their presence appears to have been largely missed by the prediction methods currently used on the genomic sequence. Equivalent homologues could not be detected in the human, microbial, Drosophila or C. elegans genomic databases, suggesting a function specific to plants. Preliminary RT-PCR analysis indicates that at least two members of the family (SPH1, SPH8) are expressed, with expression being greatest in floral tissues. The gene family may total more than 100 members, and its discovery not only illustrates the importance of the genome sequencing efforts, but also indicates the extent of information which remains hidden after the initial trawl for potential genes.