In a field experiment the wheat cultivars Glenwari, Exchange, Thatcher, Warigo, Idaed 59, and Hopps were assessed as having slow rusting abilities when artificially inoculated with a mixture of 22 physiologic races of stem rust. Resistance that resulted in slow rusting was demonstrated in growth cabinets with adult plants of these six cultivars after standardized inoculation with three stem rust races followed by uniform incubation and growth conditions. The investigation revealed that the following different kinds of resistance were involved: (1) 'specific' resistance effective against race C17 was observed in tests with seedlings and adult plants of Thatcher, Hopps, and Idaed 59; (2) low receptivity was observed in tests with adult plants of Glenwari, Warigo. Hopps, and Idaed 59; and (3) low urediospore production per pustule was evident in tests of the cultivar Exchange. The resistance of the cultivar Thatcher appeared complex, involving slow and nonuniform pustule development.Resistance expressed as low receptivity and low urediospore productivity varied in effectiveness against different physiologic races. In many instances significant interactions occurred between cultivars and races. Temperature changes appeared to affect the expression of resistance in many race–cultivar combinations. Thus, this type of resistance does not fit the definition of horizontal resistance proposed by van der Plank. Nevertheless, field investigations indicate that the resistance is effective against many rust races and could be potentially valuable in breeding programs.