Interlock assemblies (110) for attaching a loading unit (100) to a surgical stapling instrument are provided. The interlock assemblies are formed on a proximal end of a shell member (102) and on a distal end of an adapter assembly (150). The interlock assemblies include a collar member (120) for selectively securing the shell member to the adapter assembly. The collar member may be mounted on the shell member or on the adapter assembly.
