In order to have a detailed analysis of the distribution of stresses in the lung, one needs to understand the mechanical behavior of the lung material. For the stress-strain relationship of human lung, the present state of the art is that the form of the constitutive equations is known, but associated material constants are unknown. In this study, biaxial loading experiments were done on specimens of excised cadaver lung parenchyma without the effects of large blood vessels, bronchi, and pleura. Curves of strain vs. stress were recorded. A non-linear form of strain energy function was used to examine the stress-strain relationship. This relationship fits the experimental data well. The analysis based on data from 11 specimens of excised human lung parenchyma yielded that the physical constants are C/delta = 3.06 +/- 0.84 K x dyn/cm2, alpha = 4.47 +/- 1.94, and beta = -4.20 +/- 2.55.