Aseptic loosening of cemented total hip replacements is thought to involve mechanical failure of the cement-bone interface. However, the mechanical response of the interface, particularly the post-yield behavior, is not well understood. The purpose of this study was to determine the constitutive behavior of the cement-bone interface for loading in shear using a combination of experimental and finite element methods. A total of 55 cement-bone specimens (5 x 10 x 15-20 mm) from the proximal femur of human cadavers were loaded to failure under displacement control with use of a custom shear test jig. Finite element models of the test specimens were made and included provision for a two-parameter nonlinear interface model at the cement-bone interface. The experimental tests revealed a complicated load versus displacement response with an initial linear region and a reduction in slope until the ultimate strength (2.25+/-1.49 MPa) was reached, followed by an exponential decrease in load with increasing displacement until the entire interface debonded. Failure most often occurred at the cement-bone interface, where the cement penetrated into the bone with bone remaining in the cement in 30 specimens and with bone remaining in the cement and cement spicules remaining in the bone in 22 specimens. The adjacent bulk bone and cement did not appear to be permanently deformed. Finite element models of the test specimens revealed that failure initiated at the base of the test specimen before the peak load had been reached. The two interface parameters, interface strength (2.71+/-1.90 MPa) and interface-softening exponent (4.96+/-3.47 1/mm), could be determined directly from the experimental data and provided a good fit with the experimental structural response for a wide range of interface strengths. These results show that the cement-bone interface does not fail abruptly when the shear strength is reached but absorbs a substantial amount of energy with post-yield strain-softening behavior.