Casein was grafted with acrylonitrile and with a binary mixture of acrylonitrile and n-butyl methacrylate. The influence of absorbed moisture at different relative humidities on the stress-strain characteristics was studied. Tensile properties were very much affected by the absorbed moisture in both cases. The effect produced by water was very much pronounced in the case of casein grafted with acrylonitrile. Although the absorbed moisture level was the same in both grafted caseins, due to the influence of n-butyl methacrylate chains, which reduces the T-g of the film, the tensile properties and fracture behavior were very different from the films of casein grafted with acrylonitrile. Due to the application of stress in the lateral direction, tensile strength and elongation at break in the longitudinal direction was reduced. Both systems show that after an applied lateral stress of 5 X 10(-2) Mpa the variation of stress and strain was not characteristic and the value levels off after that level.