The lower-bound initiation toughness of A533-B reactor-grade steel was determined over the temperature range from 3 to 50°C. The toughness of the steel was depressed toward the lower-bound value by using the following testing procedures: (1) dynamic loading, (2) notched-round-bar specimens, and (3) axial precompression of the notch. The paper describes in detail the method of applying the impact load to the specimen, the method of precompressing the specimens, and the testing procedure. The dynamic initiation toughness, KId, which correlates with the lower-bound toughness, was determined from analysis of the strain-time behavior of the specimens. Also, the results from a fractographic analysis were correlated with those from the straintime analysis. The lower-bound toughness from this study compared favorably with KIa and KId data established through more extensive testing programs.