The POU (Pit-Oct-Unc)-domain transcription factor, Oct-4, has become a useful marker of pluripotency in the mouse. It is found exclusively in mouse preimplantation-stage embryos after embryonic genome activation and is a characteristic of mouse embryonic stem (ES) cells, and its absence in knockout mice precludes inner cell mass (ICM) formation in blastocysts. Expression of Oct-4 has also been associated with pluripotency in primate cells. Here, we undertook a systematic study of Oct-4 expression in rhesus macaque preimplantation embryos produced by intracytoplasmic sperm injection and in ES cells before and after exposure to differentiating conditions in vitro. We also evaluated Oct-4 expression as a means of monitoring the extent of reprogramming following somatic cell nuclear transfer. Oct-4 was detected by reverse transcription-polymerase chain reaction and immunocytochemistry with a monoclonal antibody. Monkey pronuclear-stage zygotes and cleaving embryos up to the 8-cell stage showed no detectable Oct-4. Nuclear staining for Oct-4 first became obvious at the 16-cell stage, and a strong signal was observed in morula and compact morula stages. Both ICM and trophectodermal cell nuclei of monkey early blastocysts were positive for Oct-4. However, the signal was diminished in trophectodermal cells of expanded blastocysts, whereas expression remained high in ICM nuclei. Similar to the mouse, hatched monkey blastocysts showed strong Oct-4 expression in the ICM, with no detectable signal in the trophectoderm. Undifferentiated monkey ES cells derived from the ICM of in vitro-produced blastocysts expressed Oct-4, consistent with their pluripotent nature, whereas ES cell differentiation was associated with signal loss. Therefore, Oct-4 expression in the monkey, as in the mouse, provides a useful marker for pluripotency after activation of the embryonic genome. Finally, the observed lack or abnormal expression of Oct-4 in monkey nuclear transfer embryos suggests inadequate nuclear reprogramming.