ABSTRACTBy subjecting F1 (CBA × C57/BL) × A eggs at the time of 2nd cleavage to 10 μg/ml of cytochalasin B (CB), tetraploidy was produced in 52 % of 2-cell eggs and 35 % of 3-cell eggs. 2n/4n mosaic embryos were produced from 2-, 3- and 4-cell eggs and amounted to 20 % of all treated eggs. 80 % of tetraploid embryos developed in vitro into regular blastocysts with half the cell number of control diploids. The effectiveness of CB in producing tetraploid embryos is limited by the asynchrony of 2nd cleavage, both between eggs and between sister blastomeres. Two-cell presumed tetraploids were transplanted to recipients and examined between the 6th and 11th day of pregnancy. Up to days development is normal and most embryos form egg-cylinders. At days the embryonic part of the cylinders is underdeveloped and in later development fails to form an embryo. Development of foetal membranes is much less affected and in the most successfully developing egg-cylinders their formation can be fully accomplished. Failure of embryonic development appears to be due to subnormal activity of the primitive streak, resulting in shortage of mesoderm. Postimplantation development of 2n/4n mosaics was normal. While in embryos tetraploid cells were either absent or in very low proportion (below 4 %), their contribution to the foetal membranes amounted in some cases to up to 50 °/o . Elimination of tetraploid cells from mosaic embryos suggests that they have a lower proliferation rate than diploid cells.