In order to resolve the mechanisms and reasons of cellular fragmentation it is crucial to understand what genes may be responsible for regulation of this process. We report herein that human oocytes and preimplantation embryos possess abundant levels of transcripts encoding cell death suppressors, Mcl-1, Bcl-x and Bag-1, and the cell death inducer genes, Bax and Caspase-2. Lower but detectable levels of mRNA expression for the Bfl-1/a1, Bcl-w, Harakiri (Hrk) and Caspase-3 genes were also detected during all developmental stages. We also performed analysis of gene expression in single human embryos exhibiting various degrees of fragmentation at the 2-, 4- and 8-cell stages. At the 4-cell stage, embryos displaying 30-50% fragmentation showed a significant increase in Hrk mRNA levels (P = 0.016). Immunostaining with anti-Hrk antibody confirmed increased staining in some, but not all, fragmented embryos. While Caspase-3 transcripts were elevated in both 4- and 8-cell embryos exhibiting a severe degree of fragmentation, this difference did not reach statistical significance. However, accumulation of Caspase-3 mRNA in fragmented embryos was paralleled by an induction of Caspase-3-like activity. These findings suggest that cellular fragmentation in a subset of human preimplantation embryos could be regulated by certain components of a genetic programme of cell death.