Results of a laboratory study are presented which was done to investigate resistance of a two-story masonry building system to repeated and reversed lateral forces. The test structure was constructed at full scale of reinforced concrete masonry, and consisted of two symmetrical C-shaped walls with window openings. Design of the specimen was based on the newly revised Chapter 24 of the 1985 Uniform Building Code for masonry structures in Seismic Zones III or IV. The measured hysteretic character of the test structure is discussed in terms of present engineering methods for estimating strength and deflection of masonry building systems.