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The effects of fast neutron irradiation on the mechanical properties of pure iron, iron alloyed with carbon, and iron alloyed with chromium were studied as part of an investigation of irradiation damage in body-centered cubic metals. In addition, A 212 grade B and modified HY80 steels were also analyzed. It was observed in the pure iron that the yield strength was no longer a function of the grain size after a fast neutron exposure of 2 x 1018 nvt. The postirradiation yield strength appears to be proportional to the log 1/T, and furthermore neutron exposures of 2 x 1018 nvt or greater produced a sharp ductile-to-brittle transition point with a shift of 85 K in fine-grained pure iron. For each initial grain size there exists a fast neutron exposure level above which the flow curve becomes linear with the apparent elimination of the initial rapid work hardening portion of the curve. While carbon additions of up to 0.11 weight per cent increased the strength and decreased the ductility of pure iron before irradiation, these differences in properties after irradiation were not found. The addition of chromium, however, decreased the yield strength and increased the ductility of both unirradiated and irradiated iron. A linear flow curve was obtained from the iron chromium alloy after the 2 x 1018 nvt exposure but no sharp ductile-to-brittle transition was observed. The two steels generally showed increased strength and some decrease in ductility without a sharp ductile-to-brittle transition point after irradiation. These results are explained by attributing the initial increases in strength and decreases in ductility to radiation-produced point defects. However, after a certain level of exposure is reached these defects become saturated and vacancy loops are created which tangle with slip dislocations. These tangles create a dislocation cell structure similar to that found in cold worked metals. These complex defects and the resulting structure produce the independence of yield strength on grain size and a linear flow curve in irradiated iron. A possible explanation for the effects produced by the introduction of carbon and chromium is given.