The influence of crystallinity on the mechanical properties of a bulk La-based metallic glass was investigated. The degree of crystallization in the metallic glass was systematically altered by varying the annealing time above the glass transition temperature, T-g. The elastic modulus, E, of the nanocrystalline-amorphous matrix composite was observed to increase abruptly at similar to 40% crystallinity, chi. The hardness, H, however increases only linearly with the increase in chi Instrumented impact test results show a significant decrease, by similar to 90%, in impact toughness, gamma, with the introduction of a small percentage of crystallinity. This decrease in toughness was also associated with a distinct change in fracture morphology, from a ductile vein type fracture in the bulk glass to intergranular cleavage fracture in the crystalline material. Microscopic mechanisms responsible for the observed transitions in mechanical properties are discussed.