Immuno-cross-reactivity between Cryptosporidium parvum and Eimeria spp. was studied by the indirect fluorescent antibody test (IFAT) and Western blot procedure. Thirty-seven sera from asymptomatic (non-diarrheic) cattle, with known coprological (presence-absence of coccidia) and serological data respecting C. parvum, were tested by IFAT using Eimeria oocysts as antigen. Most sera (54%) displayed immunofluorescence around the surface of the Eimeria oocysts. Simultaneously, serum samples from rabbits naturally infected with Eimeria spp. (E. magna, E. intestinalis and E. residua), but free of C. parvum infection, were used to investigate the recognition of C. parvum oocyst antigens by the Western blot procedure. Fractions in the 11.5-94 kDa range, as well as others with molecular masses over 94 kDa, were recognized by sera from rabbits. Sera collected during patency period showed low or moderate reaction with antigenic fractions in the 11.5-25 kDa range. However, 29, 58 and 71 to 75 kDa proteic fractions were moderately or strongly recognized even after rabbits finished oocyst excretion.