This paper presents a method to identify, with in vivo ultrasonic measurements, the probabilistic model for the elasticity tensor of cortical bone. The macrostructure of a cortical bone is complex and uncertain in case of osteoporosis. Such materials are outside the scope of the classical theories of porous materials. The transient response of ultrasonic bone is simulated with a simplified model for which the elasticity tensor is modeled by a stochastic field constructed using the maximum entropy principle.