Thirty female guinea fowls were placed in individual cages (conditions were similar to those used for the males). The two best freezing methods (based on in vitro assessment of frozen-thawed sperm) were tested with artificial insemination (as an in vivo qualification method). Ten females (controls) were inseminated with fresh, diluted semen, whereas the remaining 20 were inseminated with frozen/thawed semen from slow and pellet method (10 birds per protocol). Artificial insemination was done within 10 minutes after thawing thrice weekly for 3 weeks using 250–300 million spermatozoa/hen/AI. The fertility of 300 incubated eggs was checked by candling at the 10th day of incubation. The rate of the true fertility (normal and dead embryos) was determined. Embryos that had died in the oviduct were detected by propidium iodide staining of the germinal disc. Egg yolks were separated from the albumen and placed into 0.9% NaCl solution. After visual fertility examination germinal discs seeming to be infertile were removed from the vitelline membrane, put into 0.9% NaCl solution and stained on a slide with propidium iodide (PI; P4170, Sigma-Aldrich Ltd, Budapest, Hungary). In case of fertile egg, propidium iodide (DNA-specific fluorescent dye of red color) stains the nucleus of the dividing embryonic cells which appear in lighting points [21].