MANY angiosperms employ self-incompatibility systems to prevent inbreeding1,2. The simple genetics of such systems3-6 have made them attractive models of plant cellular communication. Implicit in the single locus genetics is that only one or a few gene products are necessary for recognition and rejection of incompatible pollen. Results in the sporophytic system of the Brassicaceae suggest that different S-locus products are responsible for the pollen and pistil parts of the recognition and rejection response7,8. In solanaceous plants, which have a gametophytic self-incompatibility system, the S-locus product responsible for the pollen portion of the interaction has not been identified, but ribonucleases encoded by the S-locus (S-RNases) are strongly implicated in the style part of the recognition and rejection reaction9-14. In Nicotiana alata, pollen r cognition and rejection occur if its S-allele matches either S-allele in the style. The putative stylar S-RNase is abundant in the transmitting tract15, and pollen rejection may be related to action of S-RNaSe on pollen RNAs10. Efforts to understand the molecular basis for pollen recognition and rejection have been limited by the lack of a system for manipulating and expressing S-RNases4-6,16. Here we use the promoter of a style-expressed gene from tomato to obtain high levels of S-RNase expression in transgenic Nicotiana. Recognition and rejection of N. alata pollen S-alleles occur faithfully in the transgenic plants. Our results show that S-RNases alone are sufficient for pollen rejection in this system.