When a spherical wave is incident on a spherical interface of two different elastic-plastic, rate-independent materials, which of the many different admissible cases of reflection and transmission will actually occur must be determined in order to extend any numerical solution for subsequent times. An analytical method for this determination in terms of the known solution for times just prior to the incidence of the wave is outlined. The wave considered may be either an acceleration wave or a shock wave. The discontinuity conditions across the wave fronts and the continuity of displacement at the interface form the basis of this method and examples are given for illustration.