Cytogenetic sexing by means of chromosome C-banding was carried out in 1/2 and 2/4 biopsied mouse embryos. Chromosome spreads were obtained in 85.0% of 1/2-embryos and in 77.5% of 2/4-embryos. Sexing was accomplished in 30.0% of 1/2-embryos and 52.5% of 2/4-embryos, with a loss due to poor technique of 12.5 and 15.0% respectively. Transfer of sexed embryos resulted in a low implantation rate. The two live fetuses obtained were of the predicted sex.