Deoxyguanosine kinase and thymidine kinase 2 are responsible for catalysing the first step in the salvage of deoxynucleosides in mitochondria. These enzymes also play an important role in activating several antiviral and anticancer nucleoside analogs, which may lead to unwanted side-effects when the resulting nucleotides are incorporated into the mitochondrial genome. We studied deoxyguanosine kinase in submitochondrial fractions from human placental mitochondria. It was localized in the mitochondrial matrix fraction by Western blotting using a purified polyclonal antibody. This antibody was also used in an immunohistochemical in situ experiment with human embryonic kidney 293 cells, in which the deoxyguanosine kinase antibody colocalized with a mitochondrion-specific fluorescent probe and there was no significant cytosolic staining.