Mouse preimplantation embryonic cleavage rate is dependent upon the presence or absence of the Preimplantation-embryo-development (Ped) gene; which is linked to the Qa-2 subregion of the H-2 complex. Expression of Qa-2 antigens by fast developing mouse embryos correlates with Ped gene pheno-type: Qa-2^a. It is not known if the Ped gene (Qa-2^a) participates in cell differentiation in the preimplantation mouse blastocyst. Therefore, the study objective was to determine the differentiation of cells to the inner cell mass (ICM) and trophectoderm (TE) in Qa-2^a positive (Ped ) and Qa-2^a negative (Ped −) mouse blastocysts. One-cell stage embryos were recovered from the excised oviducts of PMSG (5 IU) and hCG (5 IU) primed virgin female (3–4 weeks) BALB/cByJ (Qa-2^a: Ped −) and BALB/cJ (Qa-2^a: Ped ) mice mated to fertile males (12 weeks). Embryos were collected, 14 hr after hCG, and cultured in modified α-MEM, to the hatched blastocyst stage in an atmosphere of 5% CO₂ in air, 95% relative humidity at 37°C. Cell differentiation was determined by differential staining (bis-benzimide and propidium iodide) and fluorescence microscopy. Data were analyzed by Students t-test. There was no significant difference in total cell number between BALB/cJ (mean 139) and BALB/cByJ (mean 143) embryos. A significant difference (p < 0.001) was found in the number of cells differentiating to the ICM between BALB/cJ (mean 59.0) and BALB/cByJ (mean 29.0) mouse embryos. The number of cells differentiating to the TE, between BALB/cJ (mean 80.0) and BALB/cByJ (mean 114) embryos, approached significance (p = 0.062). The results suggest that the Ped gene (Qa-2^a) may have an influential role in preimplantation blastocyst cell differentiation. Additional studies are warranted to further elucidate the role of the Ped gene in preimplantation embryo development and blastocyst formation.