We report on the birth of 39 mice following single cell embryo biopsy and precise sex determination following in-vitro fertilization. Polymerase chain reaction was used to amplify fragments of the mouse testis-specific gene sequence (pYMT2/B) on the Y chromosome and the ovary-specific gene (ZP3) sequence on chromosome 5 from the single biopsied cell. Embryo biopsy was not associated with any deleterious effects.