An equation has been derived which will permit plane-strain fracture toughness, K_Ic, to be calculated from a knowledge of uniaxial tensile properties. After corrections for constraint and strain hardening the plane-stress rigid plastic crack opening displacement expression was found to describe accurately experimental crack opening displacement for the plane-strain condition. The crack tip strain distribution was measured and found to be compatible with an r⁻¹ strain distribution within a small region ahead of the crack tip. A length parameter was identified and shown to be proportional to mean-free ferrite path in steels. From the proposed behavior of crack tip instability, it was possible to better understand the observed trend of decreasing fracture toughness with increasing yield strength and how this trend can be altered by control of the microstructure.