Minimizing the amount of code that must be written and maintained is particularly critical in the development of the user interface for a highly interactive system, since the code for the user interface represents a substantial part of the application. This is especially important where the interactive system is available on a number of distinct platforms. Providing a single user interface abstraction requiring only one set of source code that can be mapped automatically into specific interface systems appears to be the preferred approach; but the underlying model must be designed carefully in order to keep the system relatively simple, easy to use and maintain, and allow ease of experimentation as user interfaces are produced. We describe the design and implementation of IUP/LED, a portable user interface toolkit that we believe has these properties. The toolkit is designed for rapid prototyping and modification, to provide a look-and-feel appropriate to a specific computing environment, is easily expanded to support new interface developments, and supports an abstract layout model. We also present a summary of the experiences in using the toolkit to indicate that it does support the original design objectives. [ps]