The transcription factor OCT-4 is regarded as a critical factor in controlling mammalian early embryonic development because of its role in toti-/pluripotency. In human preimplantation embryos, OCT-4 studies are limited to RNA analysis of abnormally developing embryos. This study thoroughly investigated the expression pattern of OCT-4 throughout the human preimplantation development. Expression was examined by single-cell RT-PCR or indirect immunocytochemistry in 36 single oocytes of various maturity and 112 normally developing preimplantation embryos at the level of single blastomeres, morulas, blastocysts, or inner cell mass (ICM) and trophectoderm (TE) samples. Oocytes and cleavage stage embryos revealed a variable OCT-4 expression pattern, concomitant with a pure cytoplasmic localization of the protein. During compaction, the variability in expression faded away indicating embryonic OCT-4 expression and the protein appeared in the nucleus implying biological activity. In blastocysts, OCT-4 transcripts and proteins were present in the ICM and the TE. At protein level, blastocysts displayed different spatial expression patterns within a cell for the splice variants of OCT-4, which may endow them with different functional properties. As OCT-4 transcripts were also found in various differentiated cells, the presence of OCT-4 transcripts or proteins may not be sufficient for identifying undifferentiated cell lines in humans. Further, we suggest to examine the localization of OCT-4 proteins within a cell rather than to look for the presence and/or amount of transcripts.