Experiments have been conducted to determine the degree to which the mature-plant reaction of Hope wheat to race 21 of P. graminis Tritici Erikss. and Henn. is affected by variation in light intensity, length of daily light period, temperature, soil moisture, and supply of mineral nutrients. All these factors gave evidence of influencing rust reaction. A 60% reduction in light intensity during the whole growing period of the plants tended to influence their rust reaction in the direction of increased susceptibility and caused a modification in the morphology of the plants in that they were softer, less rigid, and higher in moisture content than plants grown in full daylight. The shift towards susceptibility was, however, not very marked, and in only one experiment did the reaction of the plants approach complete susceptibility. Reduction in length of daily light period also influenced the rust reaction of the plants towards susceptibility. Plants receiving six hours of light daily were less resistant than plants receiving ten hours of light and these, in turn, were slightly less resistant than plants receiving full daylight for the period of the experiment (February to June). Experiments on the effect of temperature on the reaction of Hope wheat have shown that constant high temperature (75° to 80° F.) is capable of causing a partial or even complete breakdown of mature-plant resistance. Some evidence was secured that abundance of soil moisture and of mineral nutrients diminished rust resistance.