An unusually small (8 kD) protein (p8MTCP-1) is coded by the putative oncogene MTCP-1 (also called c6.1B), involved in the translocation t(X;14)(q28;q11) associated with some mature T-cell proliferations. Here, we show by subcellular fractionation and by confocal microscopy that this protein is located in the mitochondria. This localization orientates toward a role of p8MTCP-1 in the mitochondrial metabolism which may be relevant for the oncogenic process.