This paper summarizes the results from a round robin test program that was conducted from 1983 through 1985 to evaluate a proposed ASTM standard test method for determining the crack arrest fracture toughness of ferritic materials. The round robin attracted a total of 27 participants from the United States, Canada, Europe, and Japan, each of whom agreed to test three specimens of A514 bridge steel, three specimens of A588 bridge steel, and six specimens of A533 Grade B, Class 1 reactor pressure vessel steel. Twenty-one participants had completed their testing and forwarded test results when this paper was written, which provided a data base consisting of 54 test results for each of the bridge steels at −30°C, 70 test results for the reactor steel at 10°C, and 65 test results for the reactor steel at 25°C. This paper summarizes the test procedure followed in the round robin, discusses the results obtained, and provides the rationale for the modifications to the test method that were recommended in light of the experience gained from this test program. The test procedure has since been revised and is now proposed as the ASTM Test for Determining the Plane-Strain Crack-Arrest Fracture Toughness, K_Ia, of Ferritic Steels (E 1221).