Chlamydia trachomatis is a major cause of sexually transmitted disease, infertility and reactive arthritis in the Western world, and of trachoma in the developing world. There is evidence that the chronic inflammatory reaction seen in diseases associated with chlamydiae represents a delayed-type hypersensitivity response to chlamydial antigens. Little is known about which chlamydial antigens elicit T-cell responses yet such information could have important implications in terms of both immunopathological understanding of these diseases and immunoprophylaxis design. In this study, 61 chlamydia-specific T-cell clones have been produced from the synovial fluid of an individual with sexually acquired reactive arthritis (SARA). Ten clones have been characterized in detail and used to identify T-cell stimulatory antigens of chlamydiae by means of T-cell immunoblotting. Two distinct antigenic fractions have been identified, one recognized by three of the clones (molecular weight 18,000), the other recognized by six of the clones (molecular weight 30,000). The fractions are distinct from the major outer membrane protein, the 57,000 MW stress protein and the 60,000 MW cysteine-rich membrane protein of chlamydiae. The major histocompatibility complex (MHC) restriction of the response to these antigens differed: clones recognizing the 18,000 MW antigen required antigen-presenting cells expressing DR1 subtype DRB1*0101 or DRB1*0102 which only differ at amino acids 85 and 86 on the DR beta-chain; by contrast clones recognizing the 30,000 MW antigen were presented to only by antigen-presenting cells from DRB1*0101 individuals, reflecting extreme sensitivity of these clones to the polymorphism at positions 85 and 86 on the DR beta-chain.