A high molecular weight ("HMW") protein of Chlamydia, the amino acid sequence thereof, and antibodies that specifically bind the HMW protein are disclosed as well as the nucleic acid sequence encoding the same. Also disclosed are prophylactic and therapeutic compositions, comprising the HMW protein, a fragment thereof, or an antibody that specifically binds the HMW protein or a portion thereof, or the nucleotide sequence encoding the HMW protein or a fragment thereof, including vaccines.