Among the monoclonal antibodies (mAb) against Toxoplasma gondii, mAb Tg485 specifically reacted with an 82-kDa cytoplasmic protein of tachyzoites. The protein was secreted from extracellular tachyzoites, but was not released into the parasitophorous vacuole after invasion. The cDNA fragment encoding the protein was obtained by screening a T. gondii cDNA expression library with Tg485. The full-length cDNA was amplified by the 5(')-RACE method and sequenced. The deduced amino acid sequence of the 82 kDa protein reacting with Tg485 revealed a polypeptide of 708 amino acids showing significant homology to the heat shock protein 90 (HSP90) family of other organisms, especially to those of apicomplexan species. Treatment with geldanamycin, a drug known to interfere with HSP90 function, did not affect the secretion of TgHSP90 from extracellular tachyzoites, but the entry of the tachyzoites into host cells and the intracellular growth of the parasite were significantly disturbed.