The higher rate of embryonic loss in nuclear transfer compared to in vitro produced embryos may be due to chromosome abnormalities that occur during preimplantation in vitro devel- opment. Because little is known about ploidy errors in nuclear transfer embryos, this was ex- amined using embryos reconstructed from in vitro produced embryo donors. In vitro matured oocytes were enucleated and then activated using calcium ionophore A23187 followed by 6- dimethylaminopurine (6-DMAP). Subsequently, embryos were reconstructed using blas- tomeres from day 4–5 in vitro produced donors. The embryos were cultured until day 7 at which time blastocyst nuclei were extracted and chromosome abnormalities were evaluated by fluorescent in situ hybridization using two probes that bind to the subcentromeric regions on chromosomes 6 and 7. In 16 nuclear transfer blastocysts generated from 5 donor embryos, 53.8 6 20.2 (mean SD) nuclei/embryo were examined. Of these 16, 7 embryos (43.8 were potentially abnormal because in these, 1.1 1.4 5.3 7.5 26.3 30.4 and 66.2% % of the nuclei had a chromosome composition deviating from the diploid condition, indicating a wide degree of variation between embryos. These errors comprised mainly triploid (8.2 6 10.3 [0–26.3]: SD [range]) and tetraploid (10.6 6 19.9 [0–54.9]) nuclei with other ploidy com- binations accounting for only 0.9 6 2.1 [0–2.1]% of deviant nuclei. The proportion of com- pletely normal nuclear transfer embryos was no less than those produced by in vitro fertil- ization but the distribution of chromosome abnormalities was different (p 5 0.0002). In conclusion, nuclear transfer embryos reconstructed using blastomere cells can produce over 50% blastocysts with a diploid chromosome complement. However, the contribution of chro- mosome abnormalities to embryonic loss in the remaining embryos deserves further investi- gation