Histological staining and counting of blastocyst inner cell mass icM) and trophectderm(TE) cells differentially with chromatin-specific dyes is a more accurate indicator of cultured blastocyst quality and normality than total cell number assessment. The aim of this study was to test the encctiveness of a simplified method of chemically defined differential blastosyst staining. The TE of cultured mouse and bovine blastocysts of different developmental stages was stained when blastocysts were treated with a permeabiluing solution containing the ionie detergent Triton X-100 and the fluorochrome propidium iodide. Blastocysts were then incubated in a second solution containing 100% cthanol(for fixation) and the secondary fluorochrome bisbenzimide. Fixed and stained whole blastocysts were mounted and assessed for cell number using ultraviolet fluorescent m roscopy. Using this method, in-vitro cultured mouse blastocysts(day 4.5) were shown to have an ICM:TE ratio ol 12.63 with an average total cell count of 753 t 3. While day 7 and 8 in-vitro produced bovine blastocysts were shown to have an ICM TE ratio of 13.42 and 13.36 with an average total cell count of 151.3 t 5.48 and 217.8 8.75 respectively. Blastocyst staining pattems indicate that this modified technique represents a simple and reliable altemative to current bishromatic blastocyst staining techniques for the differential assessment of cell numbers and may be useful for the assessment of blastocysts derived from in vitro maturation. novel culture systems and advanced reproductive technologies such as cloning.