To ensure that the implementation of a communication protocol conforms to its specification, a protocol test process is invoked to check for conformance. In this paper, an incremental protocol test method is proposed. The incremental protocol test method is based on modifying test architectures and enhancing Formal Description Techniques' (FDTs) functionalities so that they are powerful enough to be used in both the design phase and the test phase. To provide for direct control and observation, the proposed method has (1) a single high level formal representation mechanism, which is currently the OPS5 production system, to be used in both the design phase and the test phase; (2) a Test sequence Generator and state Monitor (TGM) that is added to the local environment. In this way, protocol test sequence generation is formally represented in production rules by combining those related elements in the formal protocol specification with additional elements used for controllability and observability in the test process. The inter-communication between the module of test sequence generation and external testers is specified abstractly and its realization in procedure-based computations relies on the host environment of the Implementation Under Testing (IUT) and external testers.