Cysts of Giardia lamblia can be a contaminant of water and food that can infect humans and cause the zoonotic disease giardiasis. Irradiation by a cobalt-60 source was investigated for its ability to inactivate cysts such that they would not be infective to gerbils. Cysts that received doses of radiation ranging from 0.25 to 2.0 kGy, with a recovery period of 6 hr or less, did not infect gerbils. Cysts that were irradiated at 0.25 kGy and allowed to recover at 23 C in water for 24 hr or longer were infective. Temperature had an effect on the ability of cysts to regain infectivity in that cysts allowed to recover at 4 C after irradiation were not infective. Therefore, recovery time and temperature are important parameters for determination of the doses that are necessary for inactivation of this pathogen. The ability of staining with propidium iodide to predict infectivity of cysts was tested. Cysts that were damaged by higher doses of radiation and were not infective to gerbils stained similarly to viable cysts that were infective. Thus, propidium iodide is not a reliable indicator of viability and infectivity of cysts that have been treated with gamma radiation.