Mice inoculated at 5, 21 and 28 days of age with 10(6) or 10(7) Cryptosporidium parvum oocysts became infected but did not exhibit any clinical signs of disease. Specific IgA antibodies were detected in faecal extracts from all infected mice by an indirect immunofluorescent assay. These antibodies first appeared between 11 and 37 days post-infection (dpi) and persisted until the end of the experiment at 55 dpl. They appeared earlier in older mice than in newborn mice. Reduction and resolution of oocyst shedding was not directly related, however, to IgA antibody levels in infected mice. Reactive C. parvum antigens were identified by immunoblotting techniques using faecal and serum samples from infected mice. IgA copro-antibodies reacted specifically with two antigens of 26 and 33 kDa, which were also identified by IgG antibodies in mouse serum. The role of these antibodies in the resolution of infections and the subsequent protection against challenge is unknown.