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Partially restrained connections and their interaction with the floor slab have received recent attention due to numerous failures of welded steel moment frames during the 1994 Northridge and 1995 Kobe earthquakes. After the earthquakes, it was apparent that the 'simple' connections, including the slab, served as a 'backup' structural system after the full moment connections failed. These connections significantly influenced the behavior of the lateral load resisting system. This paper reports on the results of a unique, full-scale bidirectional test on a composite connection. The connection behaved very well up to a uniaxial drift of about 2%. After that drift level, composite action began to deteriorate and the connection strength and stiffness declined. The results indicate that two previously unreported failure modes may need to be considered in the design of composite buildings if large cyclic bidirectional forces are present. One is a 'punch through' of the column web due to the force transfer from the 'pinned' side of the connection. The other is the formation of a concrete wedge as a result of the large bearing forces between the slab and steel column flange. As a result of this observation, a suggestion that the full depth of the slab be maintained for a region 150 mm (6 in.) around the joint has been incorporated in the commentary to Part II of the 2002 AISC Seismic Specification. DOI: 10.1061/(ASCE)0733-9445(2004)130:2(320) CE Database subject headings: Connections; Composite columns; Beam columns; Slabs; Seismic design; Cyclic loads.