Protozoan parasites of the genus Cryptosporidium cause a short-term, flu-like, gastrointestinal illness in immunocompetent persons and severe, persistent, life-threatening diarrhea in immunodeficient individuals. No effective therapy is available for the treatment of cryptosporidiosis in the immunodeficient host. Complete development (from sporozoite to sporulated oocyst) of a human isolate of Cryptosporidium was achieved in cultured human fetal lung cells and primary chicken kidney and porcine kidney cells. The growth of this newly recognized zoonotic agent in cell culture now provides a means of studying its behavior, development, and metabolism, and a mechanism for evaluation of potentially useful therapeutic agents.