Letrazuril is the p-fluoro analogue of diclazuril, a benzeneacetonitrile which is active against Eimeria spp., the causative agent of coccidiosis in poultry. Letrazuril is better absorbed and unlike diclazuril produces detectable plasma drug levels in humans following oral administration. In experimental studies on neonatal mice infected with Cryptosporidium parvum, oral administration of letrazuril for 4 days reduced stool oocyst counts by 95-98%, with complete clearing of oocysts from the stool in 47% of animals receiving the higher dose. Letrazuril has the potential to be an effective agent in the treatment of AIDS-related Cryptosporidiosis, without associated major toxicity. Severely immunocompromised AIDS patients with refractory cryptosporidiosis may show a modest, short-lived response to letrazuril. Letrazuril had been in phase I clinical trials for the treatment of cryptosporidiosis. However, this study was discontinued.