Casein was grafted with acrylonitrile and with a binary mixture of acrylonitrile and n-butyl methacrylate. Mechanical properties and the fracture of these films were studied at different temperatures. Due to possible transition, fracture behaviour changed ca. 60-degrees-C in both systems. The effect of the polymer (grafted chains and homopolymer) present in the system affected the morphology and the fracture behaviour of casein films grafted with binary mixtures of the monomer.