Abstract data types have proved to be a useful technique for structuring systems. In large systems it is sometimes useful to have different regions of the system use different representations for the abstract data values. A technique is described for communicating abstract values between such regions. The method was developed for use in constructing distributed systems, where the regions exist at different computers and the values are communicated over a network. The method defines a call-by-value semantics; it is also useful in nondistributed systems wherever call by value is the desired semantics. An important example of such a use is a repository, such as a file system, for storing long-lived data.