Abstract: The paper discusses the use of formal specifications for conformance testing of OSI protocols and divides the discussion in two parts: test design and tester design. A draft standard formal specification of the Class 4 transport protocol in Estelle is taken as the starting point for test design. The test design technique used to derive a conformance test suite is semi-formal based in part on the formal specification and also the informal specification. The tests obtained are expressed in natural language. In the tester design part, we discuss the distributed test architecture of ISO and give the detailed designs of upper and lower testers. It is shown that, in testing Class 4 transport implementations, a parametrized protocol implementation approach in the lower tester design, renders the tests easier to implement.