Normally fertilized human embryos were biopsied at cleavage stages on the third day after in-vitro fertilization (IVF). One or two blastomeres at the 8-cell stage were removed and co-cultured with the biopsied embryos. Embryos and blastomeres were assessed daily for morphological development until day 6, when the number of cells were counted by labelling the nuclei. In all, 53% of the biopsied embryos (25 out of 47) reached the blastocyst stage between day 5 and 6 and the proportion was the same irrespective of the number of cells removed. There was no significant difference between biopsied embryos from which one or two blastomeres respectively had been removed with regard to total cell numbers at the blastocyst stage (56.2 +/- 3.0 and 64.7 +/- 5.5), number of trophectoderm (45.4 +/- 3.5 and 44.0 +/- 5.7) and inner cell mass cells (14.0 +/- 1.2 and 16.6 +/- 1.8). Overall, 72% of the isolated blastomers divided at least once over 3 days in culture and 50% divided more than once. The mean overall cell number after 3 days in culture was 3.7 +/- 0.48 per blastomere (range 1-8 cells) if one cell was removed and 6.9 +/- 1.0 if two cells were removed. If the undivided blastomeres are excluded, the mean cell number was 4.8 +/- 0.51 and 8.3 +/- 1.0 respectively. Over this period, 55% of the blastomeres cavitated. Of the blastomeres taken from embryos that developed to the blastocyst stage, 92% divided and 76% cavitated. In those from arrested embryos, 50% divided (P < 0.002) and 32% cavitated (P < 0.003).(ABSTRACT TRUNCATED AT 250 WORDS)