An investigation was conducted to determine the effects of the variables; temperature, strain rate, and precracking fatigue-stress level, on plane-strain crack toughness values of a rate and temperature-sensitive steel. The crack toughness behavior of a 3/4-in-thick structural steel plate, which had a static room-temperature yield stress of 45,000 psi, was examined over the temperature range from −280 to 0 deg F for strain rates of 8 × 10−5/sec, 3 × 103/sec, and 1.5/sec. Crack toughness data, which were obtained from notched bend and single-edge-notched specimens, are presented for precracking conditions obtained at a maximum nominal fatigue stress of 25 percent and 50 percent of the room-temperature yield stress. The plane-strain crack toughness, KIc-values showed only a small sensitivity to changes in temperature and no effect due to changes in strain rate. The beginning of the transition from plane-strain to plane-stress conditions occurred at successive increases in temperature for increasing strain rate. The requirement of B > 2.5 × (KIc/σys)2 for plane-strain behavior of high-strength steels was also valid for this material when the yield stress was evaluated at the test temperature and strain rate. Increasing the fatigue-cracking stress level from 25 to 50 percent of the room-temperature nominal yield stress increased the apparent KIc-values. The data showed that a necessary condition for obtaining valid KIc-values is that the plastic-zone size which develops during fatigue cracking at room temperature must be less than that which occurs at the low temperatures and elevated strain rates of the KIc tests.