Blastomere transplantation into fish blastula embryos results in somatic chimeras, which generally provide null or a small proportion of gametes derived from the donor. This may partly explain why none of the ES-like cell lines established from fish embryos has contributed to the germline of chimeras when transplanted at the blastula stage. Here, we report that a moderate gamma-irradiation of recipient embryos, followed by transplantation of dispersed blastomeres, considerably enhances the proportion of donor-derived gametes (53% versus 5% in average). In fish, the resulting protocol should maximise the pluripotency level measured in vivo for embryonic cell lines and for cultured germ cells.