The formation of transgenic chimeric chickens for use as intermediates in the production of transgenic chickens requires the efficient incorporation of genetically modified blastodermal cells in chimeras. It was hypothesized that the genotype of the donor and recipient embryos used could affect the frequency and/or extent of chimerism. Because the breed of the donor and recipient embryos could affect chimerism, Barred Plymouth Rock (BR) and White Leghorn (WL) embryos were used in all possible donor/recipient combinations (BR/WL, WL/BR, BR/BR, WL/WL). Preliminary evidence also supported the hypothesis that BR donor cells were more frequently and more extensively incorporated into WL recipients of the B\sp21/B\sp21 major histocompatibility haplotype than recipients of other haplotypes. To test these hypotheses, chimerism was assessed by tracing the fate of donor cells in early chick embryos. Freshly dispersed chicken blastodermal cells were transfected with a lacZ reporter gene by electroporation and injected into the subgerminal cavity of irradiated recipient blastoderms. After 72 hours of incubation, viable embryos were recovered and stained with X-gal. (Abstract shortened by UMI.)