Schnadt‐type impact specimens of SAE 1019 steel were irradiated with 18.6‐Mev deuterons for the purpose of studying the influence of radiation on the brittle property as measured by the change in transition temperature. The effect was compared with that of two, five, and ten percent strain hardening. Microhardness studies were made to determine the extent and location of the radiation effect. An integrated flux of 29.6 microampere‐hr per cm2shifted the transition temperature from −1°C to 18°C. The embrittling action was not a linear function of dose. Hardness was increased from 180 to 380 Knoop numbers. It was found to depend on the depth of penetration as well as the integrated flux. While radiation caused an effect similar to strain hardening, the nature of this effect was different. Annealing studies showed recovery of the irradiated material occurred between 260°C and 480°C. An interstitial vacancy diffusion process was indicated. Activation energies increased as recovery progressed. Recovery of the five percent strain hardened material occurred between 315°C and 371°C and would appear to be relaxation of internal stresses by the movement of dislocations.