To study the in planta antiviral activity of a type-1 ribosome-inactivating protein from iris bulbs, called IRIP, Nicotiana tabacum cv. Samsun NN was transformed with the IRIP sequence expressed under the control of the 35S cauliflower mosaic virus promoter. Molecular analysis of the transgenic plants and characterization of the purified protein revealed that the recombinant IRIP from tobacco leaves has the same molecular structure and RNA N-glycosidase activity as the native protein from iris bulbs. The tobacco transformants show no apparent phenotypic side effects indicating that ectopically expressed IRIP is not cytotoxic for tobacco cells. No induction of PR-1 could be demonstrated in the transgenic plants expressing IRIP. The in planta antiviral activity of rIRIP was assessed using a bioassay with tobacco mosaic virus. All transformed lines showed a statistically significant lower number of lesions compared to the control plants. The fortunate combination of in planta antiviral activity and lack of cytotoxicity of the ectopically expressed IRIP in transgenic tobacco renders the iris RIP an interesting and useful model for the study and exploitation of the antiviral activity of type-1 RIPs.