Abstract: The pathogenicity of infrared irradiated oocysts of local isolate of Cryptosporidium sp. in normal and immunosuppressed mice and pathogenesis of the experimental infection were studied. In a pilot study, the medium infectious dose of oocysts for normal mice was 2x10³ oocysts and the suitable dose of infrared-rays to reduce substantially the infectivity of the oocysts was 700-750 nm at a distance of 48.5 cm and exposure time of 5 min. In apparently normal mice, clinical signs of the experimental infection appeared at the 2nd post- infection (p.i.) day, the parasite was seen in sections of the intestines at the 2nd p.i. day, pathological lesions appeared starting on the 2nd p.i. day and oocysts shedding started on the 4th p.i. day. In comparison, giving the medium infectious dose of the oocysts to immunosuppressed mice lead to more extensive clinical signs and lesions, additional appearance of the parasite in gastric pits and pyloric glands of the stomach and earlier shedding of the oocysts (3rd p.i. day). Infrared irradiation of the oocysts for 10, 15 or 20 min rendered the oocysts completely non-infective. When the oocysts were irradiated for 5 min and shown to apparently normal mice they caused only mild clinical signs which appeared after the 6th p.i. day and mild pathological lesions which appeared after the 6th day. Additionally, oocysts shedding started at the 8th p.i. day and the parasite was not seen in tissue sections up to the 10th p.i. day. When the irradiated oocysts were shown to the immunosuppressed mice, shedding of the oocysts started at the 6th p.i. day and only mild signs and symptoms were seen. Pathological lesions included necrosis and sloughing of tips of some of the intestinal villi and the infiltration of mononuclear cells in the muscularis mucosa of the colon. The parasite was not seen in any of the tissue sections.