A number of pluripotential embryo-derived EK cells were introduced into the blastocoele of haploid and diploid parthenogenetic embryos which were subsequently transferred to suitable recipients. At autopsy on day 10 or 11 of pseudopregnancy 22% of decidua in the diploid series contained somite-stage embryos while an additional 12% contained abnormal egg cylinder-like sacs. In the haploid series, 7% of the decidua contained somite-stage embryos and an additional 5% contained abnormal 'sacs'. In 'injected' diploid and haploid 'controls' in which the zonae were pierced with an empty injection pipette 3% and 0% respectively of decidua in these two series contained somite-stage embryos, while an additional 17% and 3% respectively of decidua in these two series contained abnormal sacs. GPI analysis revealed that the EK cells were incorporated into somite-stage conceptuses in only one third of the diploids and in none of the haploid embryos. Although the presence of EK cells considerably increases the chance of normal embryonic development taking place, a detectable contribution from the EK cells into the resulting somite-stage embryo is apparently not necessary. Possible mechanisms allowing successful early post-implantation development to occur in this study are discussed.