Primordial germ cells (PGCs) are the progenitor cells of gametes. Avian PGCs are located in the central region of the area pellucida at the blastoderm stage. PGCs enter the circulation soon after the formation of blood vessels in incubating eggs and eventually settle in the gonadal primordium. We have now examined exposure of chicken embryos to soft (low-energy) x-rays as a means of depleting endogenous PGCs and thereby improving the efficiency of chimera production. The blastoderm of White Leghorn eggs was exposed to soft x-rays for 0, 20, 40 or 60s before incubation. The irradiated embryos manifested delayed development at 60h of incubation. They also showed reduced numbers of circulating PGCs at stages 14 and 15 and of gonadal PGCs at stage 30. The hatchability of irradiated embryos was lower than that of nonirradiated controls. Irradiation for 20s was found to provide the best outcome taking into consideration both the restriction of PGC proliferation and hatchability. Dispersed blastoderm cells of quail (black plumage) embryos were introduced into the blastoderm of chicken embryos irradiated for 20 s or of nonirradiated embryos. The number of donor-derived PGCs was higher in the irradiated embryos than in the nonirradiated controls at stage 30. These results suggest that soft x-irradiation of chicken embryos is a feasible approach to depletion of endogenous germ cells and consequent improvement in the efficiency of incorporation of donor PGCs.