Proof-guided testing is intended to enhance the test design with information extracted from the argument for correctness. The target application field is the verification of fault-tolerance algorithms where a complete formal proof is not available. Ideally, testing should be focused on the pending parts of the proof. The approach is experimentally assessed using the example of a group membership protocol (GMP), a complete proof of which has been developed by others in the PVS environment. In order to obtain a partial proof example, we proceed to flaw insertion into the PVS specification. Test selection criteria are then derived from the analysis of the reconstructed (now partial) proof. Their efficiency for revealing the flaw is experimentally assessed, yielding encouraging results.