Spring wheat seedlings containing Lr20 and Srl5 resistance alleles were raised at 30°C, prior to inoculation with leaf rust (Puccinia recondita race 76-2.3) and stem rust (Puccinia graminis f.sp. tritici race 343-1.2.3.5,6) pathogens, respectively. Infected plants were then grown at one of seven temperatures in the range 18°-30° C and infection types were scored at 10 days post-inoculation. These results were compared with those obtained for plants raised at a pre-inoculation temperature of 18 °C. In both 18 C and 30° C pre-grown plants, a progressive increase in infection type was observed on resistant lines as post-inoculation temperature increased.However, resistant lines raised at 30°C had significantly higher infection types than plants raised at 18° C at all post-inoculation temperatures for which some degree of resistance was still evident in the plants raised at 18° C. The maximum temperature for expression of resistance was significantly higher for Lr20 than for Srl5, irrespective of pre-inoculation temperature. A lowering of the resistance expression was also evident in Sr/J-bearing lines raised at a very low pre-inoculation temperature (4°C). The effects of low pre-inoculation temperature on resistance were assessed in both winter and spring wheat lines. These results are discussed in the light of current ideas concerning the host membrane location of pathogen recognition events.