This study investigated the level of inactivation and the potential for Cryptosporidium parvum to repair following low doses (1 and 3mJ/cm(2)) of ultraviolet (UV) irradiation from both low- and medium-pressure UV lamps. Cryptosporidium parvum oocysts suspended in phosphate buffered saline were exposed to UV using a bench-scale collimated beam apparatus. Oocyst suspensions were incubated at 5 degrees C or 25 degrees C under light and dark conditions up to 120 h (5 days) following exposure to UV irradiation, to examine photoreactivation and dark repair potential, respectively. Cryptosporidium parvum infectivity was determined throughout the incubation period using an HCT-8 cell culture and an antibody staining procedure for detection. No detectable evidence of repair was observed after incubation under light or dark conditions following either LP or MP UV lamp irradiation.