1. The variation of the ductile-brittle transition temperature with grain size is determined not only by the temperature dependence of the yield strength but also by the temperature dependence of the surface energy of brittle cracks. 2. The experimentally obtained variation of the critical temperature with grain size confirms the assumption that the decisive act in the process of brittle fracture of polycrystalline metals is not the nucleation and growth of intragranular microcracks but their extension beyond the boundaries of the grain.