Aim: To assess the efficiency of a medium-pressure UV reactor under full-scale water treatment plant (WTP) conditions on the infectivity of Cryptosporidium parvum oocysts in an Naval Medical Research Institute (NMRI) suckling mice infectivity model. Methods and Results: Six/seven-day-old mice were administered orally 2-10 x 104Cryptosporidium parvum oocysts. Compared with nonirradiated oocysts, 40 mJ cm-2 UV irradiation of ingested oocysts resulted 7 days later in a 3 center dot 4-4 center dot 0 log(10) reduction in the counts of small intestine oocysts, using a fluorescent flow cytometry assay. Conclusion: Present data extend to industrial conditions previous observations of the efficiency of UV irradiation against Cryptosporidium parvum oocyst in vivo development. Significance and Impact of the study: Present results suggest that in WTP conditions, a medium-pressure UV reactor is efficient in reducing the infectivity of Cryptosporidium parvum oocysts, one of the most resistant micro-organisms present in environmental waters.