It is important to identify and to test serologically active antigens, so as to devise a cocktail of the best antigens or peptides. We searched for antigens that have serodiagnostic utility using two-dimensional fractionation of sonic extracts from Mycobacterium tuberculosis and probing with pools of sera from healthy subjects and patients with tuberculosis (TB). Reactive protein spots with patient sera were identified by tandem mass spectrometry. Three proteins, Rv0652, Rv2626c, and Rv3418c, which have not previously been described as serologic targets, were identified. Rv0652 protein among them was expressed in Escherichia coli and serum IgG antibodies against this antigen were measured in 150 patients with pulmonary TB and in 115 healthy subjects. The sensitivity and specificity were 39% and 92%, respectively. These results suggest that a newly identified protein, Rv0652 may be a valuable candidate to be included in a cocktail test kit for TB diagnosis.