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Given a test set for stuck-at faults of a combinational circuit or a full-scan sequential circuit, some of the primary input values may be changed to the opposite logic values without losing fault coverage. We can regard such input values as don't care (X). In this paper, we propose a method for identifying the X inputs of test vectors in a given test set. While there are many combinations of X inputs in the test set generally, the proposed method finds one including as many X inputs as possible, by using fault simulation and procedures similar to implication and justification of automatic test pattern generation (ATPG) algorithms. Experimental results for ISCAS benchmark circuits show that approximately 69% of the inputs of uncompacted test sets could be X on the average. Even for highly compacted test sets, the method found that approximately 48% of inputs are X.