Self-incompatibility in flowering plants is often controlled by a single nuclear gene (the S-gene) having several alleles. This gene prevents fertilization by self-pollen or by pollen bearing either of the two S-alleles expressed in the style. Sequence analysis shows that three alleles of the S gene of Nicotiana alata encode style glycoproteins with regions of defined homology. Two of the homologous regions also show precise homology with ribonucleases T2 (ref. 4) and Rh (ref. 5). We report here that glycoproteins corresponding to the S1, S2, S3, S6 and S7 alleles isolated from style extracts of N. alata are ribonucleases. These style S-gene-encoded glycoproteins account for most of the ribonuclease activity recovered from style extracts. The ribonuclease specific activity of style extracts of the self-incompatible species N. alata is 100-1,000-fold higher than that of the related self-compatible species N. tabacum. These observations implicate ribonuclease activity in the mechanism of gametophytic self-incompatibility.