The effect of UV irradiation on human-derived Giardia lamblia cysts (WB isolate, cultivated using the gerbil model, and purified to homogeneity) was assessed using a 254nm collimated beam protocol. The infection kinetics of UV-exposed cysts were directly compared to the infection kinetics of control (non-UV-exposed) cysts. This demonstrated that a UV dose at 254 nm resulted in significant inactivation of the Giardia cysts. Up to 2 log (99%) inactivation was observed at a UV dose of approximately 10 mJ cm(-2) (range 9.3-11.7 mJ cm(-2)). Higher UV doses (between 20 and 40 mJ cm(-2)) resulted in up to 3 log (99.9%) inactivation of the cysts. The actual inactivation at these dose levels could be higher, but for this study the maximum quantifiable limit of cyst inactivation was up to 3 logs (99.9%). Chemical actinometry using the Rahn potassium iodide actinometer was used to confirm the UV dose delivered. No correlation between a PI/DAPI vital dye viability assay and the infectious dose assay was observed. The vital dye viability assay demonstrated no inactivation. Future work using an alternative UV delivery systems including a demonstration-scale device is recommended and more work with a variety of isolates is warranted.