The collective and coordinated usage of distributed resources for problem solution within dynamic virtual organizations can be realized with the Grid computing technology. For distributing and solving a task, a Grid application involves a complex workflow of dividing a task into smaller sub-tasks, scheduling and submitting jobs for solving those sub-tasks, and eventually collecting and combining the results of the sub-tasks into a final result. The quality assurance of Grid applications is a challenge due to the highly distributed nature of the Grid environment in which the Grid application is deployed. This paper investigates the applicability of the Testing and Test Control Notation (TTCN-3) for testing the workflows of distributed Grid applications. To this aim, a case study has been created that consists of a distributed Grid application which includes a typical Grid application workflow; as the main contribution, this case study contains a corresponding distributed TTCN-3 test suite that tests the correct execution of the Grid application workflow. To demonstrate the adaptation of the abstract TTCN-3 test suite to a specific Grid environment, corresponding reusable test adapters have been implemented for the Grid middleware Globus Toolkit 4 (GT4). The realized test system demonstrates that TTCN-3 is applicable for testing the workflow of distributed Grid applications.