Glassy shell-rubbery core polymer particles, 0.1 to 0.2 mum in diameter, increase the plane strain J(IC) values of 3.175 mm thick polycarbonate from 3.05 kJ/m2 (no particles) to 9.5 kJ/m2 (7.5 phr particles) at 23-degrees-C. Some modest decreases in these values are caused by tests at -20-degrees-C in samples 9.525 and 3.175 mm thick. If only the particle concentration is varied, J(IC) increases monotonically to 7.5 phr (by wt), then levels off or decreases slightly at 10.0 phr. The total volume of 1 to 2 mum diameter cavities formed in the matrix by the apparently unbonded particles behaves similarly; cavity volume and J(IC) are directly related. With increasing particle concentration the tensile modulus is unchanged, the yield stress and strain decrease modestly, the strain at fracture decreases appreciably, and the heat recoverable orientation in the fractured samples decreases. Few cavities are formed. The particles reduce the extent of shear deformation in the tensile samples.