We have isolated 26 monoclonal antibodies which specifically recognize the extreme apex of Toxoplasma gondii, a protozoan parasite which attaches to and invades host cells via its specialized apical end. The unique apical organelles which define the phylum Apicomplexa are thought to be involved in mechanical and enzymatic aspects of invasion. Immunoblots, immunofluorescence morphology, and immunogold labeling define six classes of apically localized antigens recognized by these antibodies. Three of the classes are detergent-insoluble and localize to the conoid and the cytoplasmic face of the apical membrane, suggesting that they may be part of the parasite's membrane cytoskeleton. The remaining three classes extract with detergent and are associated with internal membrane bounded vesicles (micronemes and the upper necks of rhoptries). One class of micronemal antigens appears to be cell cycle regulated. This antigen localizes to the cytoplasm, especially the perinuclear region, in thin (recently replicated) parasites, but is apical in larger parasites VSports手机版. .