The aim of this study was to try establishing mouse ES cell lines from the early developmental stage. Fifty-two uncompacted 8-cell stage embryos were dissociated and single blastomeres were seeded on primary embryonic fibroblasts in DMEM/F12 completed with 10% foetal calf serum, 10% new born calf serum. 10(-4) M beta-mercaptoethanol. After approximately 5 days of culture, multiple cell clones exhibiting stem cell morphology grew out and were dissociated. One cell line was established (MSB1) and characterised. The karyotype and the G-banding revealed a male diploid cell line. MSB1 cells were injected into syngenic mice and produced teratocarcinomas. Detailed histological examination of the tumours showed a great variety of cell types including representatives of all three primary germ layers. Several nests of undifferentiated stem cells were also present. Microinjections of MSB1 cells into 52 blastocysts produced 2 chimeras, 1 male and 1 female. These results demonstrate that a highly pluripotents ES cell line can be derived from 8-cell stage mouse embryos. However, the male chimera appeared sterile. More experiments would thus be necessary to prove that the cell line obtained is capable to colonise the germ line.