The paper investigates numerically the influence of masonry infills properties on the seismic response of RC frame structures.A five-story plane RC frame with infill panels (benchmark), part of a residential building, situated in Piatra Neamț, was considered for this aim.The benchmark was designed according to the current design practice and based on current Romanian seismic design codes, thus the infills were modelled as uniformly distributed gravitational loads on beams.Two more cases of the same structure were examined: case 1, with infill panels made of hollowed ceramic blocks and case 2, with infills made of AAC.In both cases, infills panels were modelled through diagonal struts.Comparative nonlinear time history analyses of the structures were performed.In both cases, compared to benchmark, the results show that peak inter-story drift ratios, peak bending moments of columns and beams were reduced significantly, peak axial forces of columns had a small variations and base shears were slightly increased.