The effect of rubber particle size on fracture toughness and tensile properties have been investigated using styrene-acrylonitrile as a matrix. Pre-formed particles with poly(butyl-acrylate) core and a poly(methylmethacrylate) shell, ranging from 0.1 to 0.6 mu m in diameter, were used as a toughening agent. The morphology was checked by means of transmission electron microscopy. The test methods involved an un-notched uniaxial tensile test, notched Izod impact and notched tensile testing. The experiments were carried out with varying deformation rates and temperatures. N.m.r. experiments were used to measure network densities in the rubber core of the various particles. Uniaxial tensile tests showed that elastic modulus and yield stress of the blends were independent of particle size and network density of the rubber core. There were, however, some differences in cavitation resistance caused by the differences in network density. Easily cavitating particles produced higher toughness in notched Izod impact and notched tensile test but a clear relation with particle size could not be established for the range studied. The brittle-tough transition temperatures were much higher than for materials based on poly(butadiene) core-shell rubbers. It is suggested that the mechanical properties of the rubber particle core are the key to the toughening efficiency. It was found that high toughness could only be achieved if the particles had a low cross-link density (and a corresponding low modulus and low cavitation resistance).