1. With a completely ductile fracture, 10% prior cold tensile and compressive deformation have the same effect on the impact toughnessai of the investigated tube steels, reducing it by about 30%. In the region of the ductile-brittle transition, impact toughness may be reduced by 50–60% as a result of preliminary plastic strain, depending on the chemical composition of the steel. The reduction is usually greater after compressive deformation. Aging after tensile and compressive deformation either has no effect on impact toughness or raises it by about 10% compared to the value for steels in the deformed but unaged state. 2. The decrease in impact toughness in the region of ductile fracture after prior cold deformation occurs mainly as a result of a decrease in crack nucleation work and is to a lesser degree the result of a reduction in crack propagation work. In the mixed-fracture region, impact toughness may be reduced after cold deformation as a result of either crack nucleation or crack propagation work. The latter may increase in individual cases after tensile deformation. Aging additionally reduces crack nucleation work, but the effect of aging on crack propagation work depends on the type of deformation (tension or compression) and on the character of fracture (ductile or mixed). 3. Resistance to the growth of a brittle crack (the parameter KId) is reduced as a result of tensile deformation and increased by compressive deformation. Aging after preliminary deformation leads to practically the same value of KId for the deformed and undeformed states. 4. Crack propagation workap can be used only as an approximate characteristic of resistance of brittlecrack propagation. To accurately evaluate resistance to the growth of a brittle crack, the parameter KId should be used.