We used the spontaneously differentiated human intestinal epithelial cell line Caco-2 to develop an in vitro model of Cryptosporidium sp. infection. The mean cell infection rate was 3% +/- 2%. Asexual stages of cryptosporidia were observed on day 2 postinoculation. Transmission electron microscopy showed the presence of macrogametes at day 5. This cell line appears to be suited to the study of the mechanisms by which biological agents inhibit both sexual and asexual development of cryptosporidia.