The viability of Giardia muris cysts was studied using the fluorogenic dyes, fluorescein diacetate (FDA) and propidium iodide (PI). Using the mouse model for giardiasis, FDA or PI stained cysts were inoculated into neonatal mice. Feces were examined at days 3, 5, 8, and 11 post-inoculation for the presence of cysts. Using 1000 FDA stained cysts as the inoculum, cysts were detected at days 5, 8, and 11 after inoculation in 9/9 mice, whereas a 50-fold greater dose of cysts produced infection at three days in 27/27 mice as well as at 5, 8, and 11 days post-inoculum. Inoculation of 40 mice with 50,000 PI stained cysts did not produce infection in any of the animals. Necropsy of mice infected with FDA stained cysts showed trophozoites within the intestine. No trophozoites were detected within animals inoculated with 50,000 PI stained cysts. These results demonstrate that FDA positive cysts are non-viable and incapable of producing Giardia infections in vivo. (Copyright (C) 1987, American Society for Microbiology.)