Colocalization of mitochondria is the first step of intermitochondrial interaction or fusion in a cell. Here, we showed colocalization between exogenous mitochondria and endogenous ones or between exogenous proteoliposomes and endogenous mitochondria in mouse fertilized eggs by confocal laser microscopy. Isolated mitochondria from mouse liver and proteoliposomes containing mitochondrial membrane were directly labeled with red fluorescent aliphatic marker, PKH26, which is incorporated into lipid membrane, and then were microinjected into fertilized mouse eggs. Exogenous mitochondria appeared to be almost colocalized with endogenous mitochondria at the 4- and 8-cell stages, when mitochondria were stained with Rhodamine 123 (green fluorescent marker). On the contrary, when liposomes consisted of soy bean phospholipid were microinjected into the eggs as a control, their localization was different from that of endogenous mitochondria. Next, the submitochondrial particles and proteoliposomes were microinjected. Both the proteoliposomes and the submitochondrial particles appeared to colocalize with endogenous mitochondria at the 4-cell stage. These results suggest the existence of a factor that makes liposomes colocalize with mitochondria. Such a proteoliposome would be useful for the development of mitochondrial gene transfer techniques.