This paper is a follow up on our previous work on intrusion detection for computer systems. It is motivated by the accelerated growth of computer viruses in computer networks. Most applications and operational systems are overwhelmed by security failures at different levels. If there are no verification and monitoring procedures in the computational system, then security services (such as access control, firewalls and cryptography) become ineffective. In this paper, we present a security system for fraud detection of intruders and misuse of a mail system in a computer network environment. Our intrusion detection model is inspired by natural immune system. This paper discusses our model, and presents preliminary results for both anomaly and operational misuse of the computer system.