Background The piglet is the only model to investigate the immunogenic relationship between Cryptosporidium hominis and C . parvum , the species responsible for diarrhea in humans. Despite being indistinguishable antigenically, and high genetic homology between them, they are only moderately cross protective after an active infection. Methodology/Principal findings Here we examined the degree of passive protection conferred to piglets suckling sows immunized during pregnancy with C . parvum . After birth suckling piglets were challenged orally with either C . parvum or C . hominis at age 5 days. Animals challenged with C . parvum had significant reduction of infection rate, while piglets challenged with C . hominis showed no reduction despite high C . parvum serum and colostrum IgG and IgA antibody. Conclusions/Significance We add these data to earlier studies where we described that infection derived immunity provides partial cross-protection. Together, it appears that for full protection, vaccines against human cryptosporidiosis must contain antigenic elements derived from both species.