The resistance of polymers, with toughening agents, to fast crack propagation under steady load conditio ns was studied, and the fracture resistance of acrylonitrile-butadiene-styrene (ABS) was compared with styrene-acrylonitrile (SAN). Small laboratory specimens were used which could be precisely loaded and allowed to stress relax before being fractured. It was important for these experiments to achieve good isolation between crack initiation and crack propagation.