Cryptosporidium hominis and parvum primarily infect intestinal epithelial cells, which, in turn, play a key role in activating and communicating with the host immune system. To determinate which genes are regulated during early infection of non-transformed human epithelial cells, human ileal mucosa was removed (from surgical specimens), placed on collagen membranes, and cultured as explants. Explant cultures were infected with C. parvum, C. hominis, or control culture medium. After 24 hrs, RNA was extracted and analyzed using Affmetrix GeneChip microarrays. Among the more prominent genes with regulated expression was Osteoprotegerin (OPG), which was increased in all of the explants at 24 hrs and further up-regulated 1.58 fold by C. parvum and 2.54 fold by C. hominis infection compared with uninfected explants. Using real time PCR, we confirmed a 3.14 and 3.79 fold increase in OPG mRNA after infection with C. parvum and C. hominis respectively.