Large mainframe installations need and want to exploit the advantages of Object Technology (OT), but without totally abandoning their legacy environments. Implementing Object Orientation in such a COBOL/CICS/DB2 environment is a challenge: there is neither language support, nor development tools, nor execution infrastructure, nor testing utilities. Yet Object Orientation can be fully implemented, and a project can still meet rigorous performance requirements and tough delivery time scales. The key is to develop an infrastructure. This paper relates the lessons learned in designing and implementing such an OO infrastructure at British Telecom.