Green fluorescent protein (GFP) has been widely used in a variety of experiments in cell biology. When cells were co-transfected with the GFP gene and the bcl-2 family genes bcl-2, bcl-x(L), and bax, mitochondria appeared to aggregate at the periphery of the nucleus specifically where GFP was expressed. Little aggregation was seen in the presence of other members of the GFP family, EGFP (enhanced GFP), ECFP (enhanced cyan variant), and EYFP (enhanced yellow-green variant). GFP but not EGFP seemed to promote cell death induced by pro-apoptotic Bax. Thus, GFP specifically promotes the aggregation of mitochondria when co-expressed with a member of the Bcl-2 family in association with apoptosis.