The identification of the inciting antigen in fibrotic hypersensitivity pneumonitis (HP) is challenging. Clinical history-taking is one of the most useful methods for this identification. The data from the clinical history-taking cannot be used to objectively evaluate the inciting antigen. The 2020 American Thoracic Society/Japanese Respiratory Society/Latin American Thoracic Association guidelines for the diagnosis of HP in adults listed serum testing for immunoglobulin G (IgG) antibodies against specific antigens as a highly important method for the identification of the inciting antigen. Although serum IgG testing is a simple and convenient test, its sensitivity in fibrotic HP is extremely low. Furthermore, it only indicates that the case has been exposed to the antigen, but does not identify the inciting antigen. The combination of history-taking and serum IgG testing for antigen identification is a complicated process. The inhalation challenge test is a useful tool for the identification of inciting antigen, because a single antigen can be inhaled. Antigen avoidance has also been used in clinical practice for cases with acute exacerbation of fibrotic HP or acute on chronic HP. Specifically, the antigen avoidance test has been used in the treatment of fibrotic HP and in the identification of the inciting antigen. Despite their limitations, these tests can be of great assistance in identifying the inciting antigen. This chapter describes the usefulness and drawbacks of both tests.