Distributed intrusion detection systems have several advantages over centralized systems, such as scalability, adaptability, and fault tolerance. A current research topic in distributed systems is self-monitoring to identify corrupted intrusion detection systems. One way of self-monitoring is for intrusion detection systems to check each other. As we describe herein, this can be done by mobile agents using an immunity-based diagnostic method modeled on idiotypic network theory. In simulations, the credibility of normal intrusion detection systems remained near 1, while it fell to about 0 for corrupted intrusion detection systems, thus enabling identification of the latter. We also confirmed what effects some parameters have on the diagnostic capability.