Masonry shear panels used as infilling in steel frames are investigated experimentally. Twenty-eight large-scale specimens were tested to ultimate panel strength under in-plane, horizontal loading applied at roof level. Of the parameters varied in the test series, interface conditions between panel edges and frame were found to significantly affect the strength and behaviour. Column-to-panel ties were found to be ineffective in increasing ultimate strength while initial stiffness was only marginally increased. A 20 mm gap between the upper edge of a panel and roof beam was particularly detrimental to the system shear capacity. While panel openings reduced initial major crack load, the same was not necessarily true for their effect on ultimate strength. Reinforced bond beams at one third and two thirds of the panel height forced initial major cracking to occur quite close to ultimate, which itself was only marginally increased. The lowest initial major cracking and ultimate loads were recorded for those specimens consisting of a panel in a hinge frame with a 20 mm gap between the upper edge of the panel and roof beam. Key words: masonry, infilled panel, steel frame, experimental, in-plane, behaviour, strength.