To perform a dynamic analysis of a nonlinear structure interacting with an unbounded medium in the time domain, it is desirable to formulate the interaction forces of the unbounded medium on the structure-medium interface using displacements. The corresponding response matrix to a unit impulse of displacements is calculated using the scaled boundary finite-element method. This method, based solely on finite elements, is very suitable to model an unbounded medium, as the radiation condition at infinity is rigorously satisfied and the discretization is limited to the structure-medium interface. (C) 1998 Elsevier Science S.A.