In human embryos, blastomeres differentiate into trophectoderm (TE) cells and inner cell mass (ICM) cells of blastocysts. Although morphologically indistinguishable, blastomeres at early cleavage stages are likely to undergo changes on a molecular level that make them destined to become ICM or TE cells. While the transcription factor Oct-4 might serve as a marker for totipotent ICM cells, human chorionic gonadotrophin might be used as the equivalent for TE cells. This study reports a reverse transcription-polymerase chain reaction procedure to assess human beta-HCG mRNA concentrations as well as ploidy in individual blastomeres from normally and abnormally fertilized human embryos. beta-HCG mRNA was detected in both euploid and aneuploid cells and in oocytes. Surprisingly, beta-LH mRNA was also detected in some euploid blastomeres. In regard to preimplantation genetic diagnosis, assessment of expression levels of beta-HCG and Oct-4 mRNA in individual biopsied cells might serve as a tool to identify embryogenic blastomeres in combination with testing for chromosome and single gene abnormalities.