The effect of unidirectional solidification on the mechanical properties of high-strength steels was investigated by means of split heats of AISI 4340 and 25 per cent nickel maraging steel. One half of these heats were unidirectionally solidified and the other half of these melts were allowed to solidify to produce an equiaxed structure. These ingots were forged and rolled to a light gage sheet 0.040 to 0.060 in. thick. The materials were evaluated by means of standard tension tests and fracture toughness studies. By means of compliance measurements, it is possible to determine the crack resistance as a function of absolute crack extension. This investigative technique was used to study the AISI 4340 steel. Unidirectional solidification did not alter the crack resistance properties of this steel. However, a 10 per cent improvement in plane-strain fracture toughness as determined by the pop-in technique was observed for the unidirectionally solidified material. In order to obtain very high strength levels, the total content of hardener elements was increased in the 25 per cent nickel maraging steels. Consequently these materials were quite brittle and the results not definitive. However, it was demonstrated that a homogeneous structure is necessary for high values of fracture toughness in this material. The data were interpreted in terms of critical crack size for instability at yield stress. Both through-cracks and part-through-cracks were considered in this analysis.