A monoclonal antibody (MAb) has been generated against a novel 63 kDa surface/apical antigen of Toxoplasma gondii tachyzoites which is identified here as TgAMA-1, the Toxoplasma homolog of Plasmodium apical membrane antigen-1 (AMA-1). Sequence analysis, phase partitioning in Triton X-114, and labeling of TgAMA-1 with iodonaphthalene azide all suggest that TgAMA-1 is a type I transmembrane protein. There is a high degree of sequence similarity between TgAMA-1 and Plasmodium AMA-1, most notably in the position of conserved cysteine residues within the protein's predicted extracellular domain. In contrast to full length Plasmodium AMA-1, which has previously been localized to the rhoptries, it is shown here by immunofluorescence and immunoelectron microscopy that intracellular TgAMA-1 is found in the micronemes. A 53 kDa N-terminal proteolytic fragment of TgAMA-1 is constitutively secreted from the parasite at 37 degrees C. As is the case with other microneme proteins, the proteolytic processing and secretion of TgAMA-1 is dramatically enhanced in response to treatments which increase intracellular calcium levels.