Monoclonal antibodies (MAB) were raised against whole sporozoites of Eimeria papillata and their organelle fractions. These fractions included rhoptries, dense granules, mitochondria and pellicle fragments. Twenty MAB recognized structures within the apical region of sporozoites as observed by indirect immunofluorescence antibody (IFA) test. Of these, 2 MAB showed a reaction with extracellular and intracellular sporozoites at the apical region as well as with after structures within invaded cells in culture (24 hr monolayers of MDCK cells). Within 60 minutes of inoculation fluorescence in the intracellular sporozoites decreased. This decrease corresponded with IFA reactions in the invaded cells in the form of cytoplasmic granules, on the host cell surface and in the vicinity of the parasitophorous vacuole. These results indicate that proteins of parasite origin are released from the apical region and are dispersed in different parts of the host cell. © 1993, Gustav Fischer Verlag Jena. All rights reserved.