To prevent any data from being accessed by unauthorized users, it is necessary for stock trading systems (STS) to use multilevel secure database management systems in controlling concurrent executions among multiple transactions. In STS, analytical transactions as well as mission critical transactions are executed concurrently, which makes it difficult to use traditional secure real-time transaction management schemes for STS environment. In this paper, we propose the read-down relationship-based secure one snapshot protocol (SOS) that is devised for the secure real-time transaction management in STS. By maintaining an additional one snapshot as well as working database, SOS blocks covert-channels without causing the priority inversion phenomenon. We introduce the process of SOS protocol with some examples, present the proofs of devised protocol, and then evaluate the performance gains by means of simulation method. (C) 2003 Elsevier Inc. All rights reserved.