Masonry infill panels in framed structures affect the strength, stiffness, and ductility. For seismic loading, ignoring the composite action is not always on the safe side, since the interaction between the panel and the frame under lateral loads changes the stiffness and the dynamic characteristics of the structure, and hence, its response to seismic loads. To study the influence of brick masonry infill panels on seismic performance of reinforced concrete (RC) frames, equivalent diagonal strut is used to model the stiffness effect of the masonry panels. The response of a bare frame is compared with the full and partial infill with centrally located opening. In general, the shear forces in the columns are decreased by the presence of infill panels. When subjected to lateral loading, the frame with full infill has better response whereas; the response of infill with very large opening is comparable with bare frames.