activity) fluoresced brilliantly, while the zona pellucida and the mucin coat did not fluoresce. The blastomeres were more sharply outlined than when observed using normal light microscopy. In blastocysts the region of the embryonic node was usually brilliantly fluorescent, but in some cases this zona was dark although the surrounding trophoblast cells were brightly fluorescent; those embryos showed no mitotic activity after culture. All embryos which did not fluoresce after recovering from the uterus showed no further development in vitro. However, some embryos (mainly in rabbits) contained both living and dead blastomeres as determined using the FDA test. These differentially reactive embryos were viable in 1 out of 6 cow embryos and in 8 out of 38 rabbit embryos; the remaining showed no mitotic activity after culture. Although quantitative data are not yet available, the determination as to whether such embryos are still viable will likely depend on the percentage of living cells. This means that embryonic death progresses gradually, which can influence the reliability of the test. So in rabbits, 72 to 84 h after ovulation, unfertilized eggs were still fluorescent as well as some of the 4-to 8-cell embryos that were slow to develop. In cattle, however, in which the time between recovery and ovulation was longer, late development stages, and the unfertilized eggs showed no more fluorescence. The short-term incubation of the embryos in the FDA medium, described here, will probably not impair their uterine development and no teratogenic effects could be seen in nineteen 16-to 20-day-old rabbit fetuses transferred as 16-cell embryos after the FDA test. Currently we are determining whether the FDA test can be used to test the viability of thawed embryos. Results of these studies will be published in more detail elsewhere.