Tensile tests have been carried out over a wide range of test speeds on compact tension specimens of polyamide 6 containing spherical silica particles, whose size and content had been adjusted to give optimum impact performance in conventional impact tests after conditioning at 50% relative humidity. The tensile test results confirmed there to be a significant improvement in the high speed crack initiation resistance at room temperature and at high moisture contents on addition of the silica particles. However, at low moisture contents and/or temperatures well below the glass transition temperature, the crack initiation resistance was reduced. It is hence inferred that for the chosen silica particle distribution, toughening requires a certain minimum level of matrix ductility in order to be effective.