Gene-specific RNA directly involved in resistance of wheat (Triticum aestivum L.) to stem rust (Puccinia graminis Pers. f. sp. tritici Eriks. and E. Henn.) was produced only in interactions involving an avirulent race of stem rust. The genotype of the host did not affect the production of this RNA, suggesting that the RNA involved in the expression of resistance is a product of the gene for avirulence.Several parameters affecting the bioassay used for the detection of this active RNA were investigated. These included temperature, infection density, the time interval between infection and injection of extract, and the time interval between injection and harvest.A comparison between the number of necrotic sites produced after injection of active RNA into leaves of the resistant line and the number produced in the incompatible interaction showed that 2–7% of the potentially reactive sites respond in the bioassay by becoming necrotic.The active RNÀ isolated from a stem rust – wheat interaction involving a given gene for avirulence produced a response in bioassay plants only when both the corresponding gene for resistance and stem-rust haustoria were present.