The fatigue life data in Fig. 4 show that the improvement in monotonic strength is negligible, whereas the fatigue life is prolonged by as much as a decade with the stronger interface by modification of the PP matrix with maleic anhydride. A corresponding shift to longer fatigue lives, together with retention of stiffness, have also been observed for a carbon-fibre/epoxy system with improved interfacial properties[16]. No distinct regions pertinent to the fatigue life diagram[1]can be discerned in Fig. 4 since all data points seem to belong to the sloping scatter band associated with a progressive fatigue mechanism. The monotonic strength is basically controlled by the fibres, hence the overlap for low cycles in the fatigue-life curve. The shorter fatigue lives of glass-fibre/PP for equal strain amplitudes suggest a more rapid growth of the critical damage site. The weaker interfacial bond must be the reason for the enhanced damage propagation rate in glass-fibre/PP. This can be confirmed by analyzing the active damage mechanisms. Such an analysis is useful since it indicates the weak microstructural member of the composite material.