This note presents a study on robust stability of two-dimensional (2-D) discrete systems in the Fornasini-Marchesini (F-M) state space setting. A measure of stability robustness of a stable F-R I model is defined. Relation of this measure to its counterpart in the Roessor state space and related computational issues are addressed. Three lower bounds of the stability-robustness measure defined are derived using an one-dimensional parameterization approach and a 2-D Lyapunov approach. A numerical example is included to illustrate the main results obtained.