A method of diagnosing Mycobacterium tuberculosis infection in a human, or of determining whether a human has been exposed to Mycobacterium tuberculosis, comprising (i) contacting T-cells from said human with one or more of (a) a peptide having the sequence shown in SEQ ID NO 20, (b) a peptide having or comprising the sequence of at least 8 consecutive amino acids of the sequence shown in SEQ. ID NO 20; or (c) a peptide having or comprising a sequence which is capable of binding to a T-cell receptor which recognises a peptide as defined in (a) or (b); and (ii) determining whether any of the said T-cells recognise said peptide, wherein steps (i) and (ii) are optionally carried out in vitro. The peptide is the product of the RV3615c gene.