Inactivation of Cryptosporidium parvum oocysts in finished drinking water by medium-pressure ultraviolet (UV) light was investigated at bench-scale using a collimated beam apparatus, and at demonstration scale using the Calgon Carbon Corporation Sentinel(TM) system. Oocyst viability was assessed using in vitro (DAPI/PI and maximized in vitro excystation) and in vivo (neonatal mouse infectivity) assays. Using the neonatal mouse infectivity assay, the bench-scale studies showed >4 log inactivation at UV doses as low as 41 mJ cm-2; the in vitro surrogate assays showed little or no inactivation at this and higher UV doses. The in vitro assays, which indicate oocyst viability, grossly overestimated the UV doses required to prevent infection by the oocysts in susceptible hosts. The demonstration studies, carried out under the NSF/EPA ETV program, provided results that agreed with the bench-scale results and showed that a UV dose as low as 19 mJ cm-2 provided 3.9 log inactivation of Cryptosporidium oocysts.