Monoclonal antibody (mAb) TxE2, reactive with Toxoplasma gondii excretory products, detects an acidic 29 kDa protein (p29) which, in 2D gel electrophoresis, exhibits a migration pattern distinct from those of the toxoplasmic excretory proteins described so far. The sequence of seven peptides from tryptic digestion of isolated p29 allowed the design of primers to obtain the coding DNA sequence. The full-length gene was amplified from genomic DNA of T. gondii strain BK and the sequence was identical with that of the corresponding cDNA, providing evidence for an intron-free gene structure. A single mRNA transcript of 1.3 kb was detected by Northern blot analysis. The deduced 236 amino acid protein contains a putative N-terminal signal peptide, one site of potential N-linked glycosylation, and, close to the C-terminus, a further hydrophobic, putative transmembrane domain. With synthetic peptides spanning the sequence of p29, the epitope for mAb TxE2 was mapped adjacent to the putative signal sequence. The antigen, which represents almost 0.5% of T. gondii protein, is expressed in strains of all three intraspecies subgroups, and is associated with the parasite dense granules as demonstrated by immunoelectron microscopy. In tachyzoite-infected cells, p29 accumulates within the parasitophorous vacuole and co-localizes with its delimiting membrane. In bradyzoite-infected cells, p29 is present within the host cell cytoplasm as detected by immunofluorescence staining, and, furthermore, in the supernatant of cyst-bearing cell culture lacking extracellular parasites as shown by enzyme-linked immunosorbent assay (ELISA). Thus, p29 which is named dense granule protein (GRA)7 may indicate the presence of intracellular toxoplasma.