Structural and biochemical differentiation of germ cell mitochondria is supposed to determine the fate and integrity of mitochondria in the early embryo. Immunofluorescent labeling of the primordial germ cell epitope 2 (PG2), which is associated with the outer mitochondrial membrane and is germ cell specific from the time of germ cell segregation during gastrulation, was used to elucidate biochemical characteristics of mitochondrial differentiation leading to a functional gamete. The PG2 epitope is found in both mitotic and meiotic male and female postnatal germ cells, but PG2 expression ceases transiently in initial stages of meiosis, i.e., in the female during early stages of follicle formation and in the male during prespermatogenesis and initial phases of spermatogenesis. Because the PG2 epitope is detectable in germ cells at the time when structurally immature mitochondria are present, we speculate that PG2 immunoreactivity closely mirrors the progress of mitochondrial differentiation during gametogenesis.