The need for an effective compound for the prevention and treatment of cryptosporidiosis in humans and animals has led to the testing of benzimidazoles based on reports that albendazole was clinically effective against related protozoan parasites causing microsporidiosis in humans. Albendazole and other benzimidazole derivatives were tested for prophylactic efficacy against cryptosporidiosis at dosage levels 1-3x the levels found effective for treatment of cattle or sheep for helminth infections. Daily dosage levels of thiabendazole, parbendazole, oxibendazole, mebendazole, and albendazole, as high as 200, 30, 10, 15, and 15 mg/kg of body weight, respectively, were not efficacious in neonatal mice. Although the number of parasites in histologic sections of intestine from mice mediated with 15 mg albendazole/kg of body weight was significantly lower than in unmedicated control mice, suggesting activity against the parasite, a high percentage of epithelial cells in the medicated mice were infected.