Chlamydia trachomatis causes sexually transmitted diseases and is associated with serious long-term sequelae such as tubal infertility and ectopic pregnancy. There have been suggestions that chlamydial antigens of approximately 57,000-60,000 Mr may be involved in the immunopathology. Two important chlamydial antigens of 57,000-60,000 Mr are a Triton X-100-soluble antigen, which induces hypersensitivity in ocular models, and a sarcosyl-insoluble cysteine-rich structural protein, omp2. In this study, a 57,000 Mr Triton X-100-soluble protein was characterized as the chlamydial homolog of groEL, a heat shock protein. Using protein fractions, antibody responses of pelvic inflammatory disease (PID) and ectopic pregnancy patients to chlamydial groEL and omp2 were differentiated. Nearly all patients in both groups were reactive to omp2. Of those with titers greater than or equal to 1:512, 31% of PID sera and 81% of ectopic pregnancy sera were positive for chlamydial groEL (P = .004). This selectivity suggests that women with PID who develop chronic sequelae are those with antibody to groEL.