The expression of the transcription factor Oct-4 is thought to be one of the decisive factors that maintain totipotency in embryonic and germ cells. In mice, oct-4 is exclusively expressed in germ cells and totipotent cells of the embryo. In humans, Oct-4 is expressed in germ cells, embryonic stem cells and whole embryos at various stages of development. However, there is limited information about the distribution of Oct-4 expression in human embryos. In an attempt to address this issue, the inner cell mass (ICM) and trophectoderm (TE) of 17 human blastocysts were separated and Oct-4 mRNA expression individually assessed by reverse transcription-polymerase chain reaction (RT-PCR). In discarded blastocysts that developed from two pronuclear zygotes, the mean Oct-4 expression was 31 times higher in totipotent ICM cells than in differentiated TE cells. This finding suggests that, in accordance with data from the mouse, Oct-4 is highly expressed in human ICM cells as opposed to TE cells; this in turn supports the hypothesis that Oct-4 plays a similar role to maintain totipotency in these two species.