Earthquake response analysis of two-bay one-story steel frames with or without concrete slabs was performed by computer-actuator on-line testing. We primarily investigated the effect of the presence or absence of a concrete slab on the elastic-plastic behavior of frames. Examination was also made of the difference in behavior between two types of composite beams, one joined to an interior column and the other to an exterior column, and of the relationship between frame ductility and beam ductility. The natural period of the frames without concrete slabs, 0. 49 sec, was predetermined in the analysis. The maximum acceleration of the El Centre NS (1940) was amplified to 1.18 or 2. 25 times the yield acceleration of the frames without concrete slabs. To obtain quantitative restoring force characteristics of the frames analyzed by the on-line testing, monotonic and cyclic loading tests were conducted on frames whose shapes were identical to those of the frames used in the on-line test. The on-line test analysis revealed that the presence of the concrete slabs greatly affected the seismic behavior of the frames. In particular, the response beavior was governed by whether a compressive failure of the concrete slab occurred or not, depending on the magnitude of the maximum acceleration.