Cryptosporidiosis has become an important infection in man, particularly among very young or immunocompromised individuals. Here, the protective role of antibody responses to Cryptosporidium was examined using a well established murine model of cryptosporidiosis. Although normal neonatal BALB/c mice exhibited good IgM and IgG serum antibody responses, no correlation could be drawn between the intensity of these responses and the severity or duration of cryptosporidiosis. Moreover, B cell-deficient (anti-mu-treated) neonatal BALB/c mice did not differ from untreated or normal rabbit Ig-treated, age-matched controls in the onset, peak, or duration of cryptosporidiosis. The apparent absence of a role for antibody in these self resolving infections was supported by the lack of susceptibility of anti-mu-treated adult BALB/c to attempted infection with doses of Cryptosporidium 10 times the dose required to infect 100% of normal (Ig producing) neonates. The results suggest that the role of specific in vivo antibody responses in the resolution of murine infection with this coccidian parasite is minor and that the likelihood of success for cryptosporidial vaccines aimed solely at enhancing in vivo antibody production may be limited.