Normally fertilized human embryos biopsied 3 days after in-vitro fertilization (IVF) have been examined for effects on viability and development in vitro after removal of one or two cells at the 8-cell stage (1/8 and 2/8) from each embryo. A high proportion of 7/8 and 6/8 biopsied and unmanipulated embryos developed to the blastocyst stage between days 5 and 6 (79, 71 and 59%, respectively), and many biopsied embryos (56%) hatched from the zona pellucida in vitro. The viability of biopsied embryos which developed to the blastocyst stage was assessed by daily non-invasive measurement of the uptake of two energy substrates, glucose and pyruvate. Uptake of both substrates was generally lower in 7/8 and 6/8 biopsied embryos but only in proportion to the reduced cellular mass. The total cell number and the numbers of both trophectoderm (TE) and inner cell mass (ICM) cells in biopsied embryos at the blastocyst stage, counted by differential labelling of their nuclei, were also reduced in proportion but the ratio of ICM to TE cells was maintained in both 7/8 and 6/8 biopsied embryos. We conclude that removal of one or two cells at the 8-cell stage, while reducing the cellular mass, does not adversely affect the preimplantation/development of biopsied embryos in vitro and suggest that this approach could be used for preimplantation diagnosis of genetic defects.