Assessing interoperability in the networked information services and applications environment presents difficult challenges due in part to the multi‐level and multi‐faceted aspects of interoperability. Recent research to establish an interoperability testbed in the context of Z39.50 protocol clients and servers and online catalog applications identified threats to interoperability and defined a question space for interoperability testing. This paper reports on follow‐up research to develop an alternative approach for interoperability testing in the context of networked information retrieval that uses specially designed diagnostic records. These records, referred to as radioactive records, enable interoperability assessment at the protocol and semantic levels. This approach appears to offer an extensible method for interoperability testing for other metadata and protocol application environments. The resulting interoperability testbed incorporates additional components to exploit automatic processes for interoperability testing and assessment, thus improving the efficiency of interoperability testing.