The asymptomatic nature of chlamydial infection renders the differential diagnosis of acute and chronic infection difficult. An untreated Chlamydia trachomatis infection can become chronic, result in disease sequelae such as salpingitis and pelvic inflammatory disease (PID), and ultimately culminate in tubal occlusion and infertility. Diagnostic tests for C. trachomatis such as nucleic acid amplification testing (PCR), antigen detection and serological methods have variable performance capabilities with respect to sensitivity, specificity and stage of infection. The use of PCR as a diagnostic tool is somewhat limited, as specimen collection is routinely sampled from the lower genital tract; hence, infections in the fallopian tube where inflammatory damage is most significant, escape detection. Furthermore, PCR can only detect selected Chlamydia DNA sequences from readily accessible sites of the genital tract, and therefore cannot differentiate between acute and chronic infection. Other serological assays aim to discriminate the various stages of C. trachomatis infection through identification of key antigens. The efficacy of these assays however is impeded due to cross-reactivity between chlamydial species and the subsequent antibody response against the target antigen is not restricted to patients with a specific stage of infection. To identify antibody responses capable of differentiating various states of chlamydial infection, samples were collected from both men and women given the variability of immune responses between the two genders. Samples were assigned to a patient group according to infection status and then probed against protein extracts of HEp-2 cells infected with C. trachomatis serovar L2 and HEp-2 cells pre-treated with IFN-γ and infected with C. trachomatis serovar L2. (persistence cell culture) Serological analysis revealed the presence of five antigens (denoted bands A, B, C, D and M) which were shown to be differential between patient groups. Identification of bands B and C by N-terminal sequencing provided two possible candidates for each antigen, ie. CT727 and CT396 (band B) and CT157 and CT423 (band C). In contrast, band M which was unique to males was a PmpB (probable outer membrane protein B) fragment. The four target antigens (CT157, CT423, CT727 and CT396) were expressed as recombinant proteins using autoinduction media and were subsequently probed by both male and female sera to evaluate their diagnostic potential. Results showed that two chlamydial antigenic targets (CT157 and CT727) have the potential to discriminate between acute and chronic C. trachomatis infection. However, since only a small number of samples (n = 3) were used for this aspect of the study, the findings should simply be viewed as preliminary. In females, sensitivity and specificity values were derived using various combinations of the four target antigens into a panel format for the purpose of detecting chronic C. trachomatis infections. The preferred format was B + C with a sensitivity and specificity of 80% and 84% respectively. Using the IFN-γ-mediated persistence model, only two of the five antigenic targets were shown to be differentially expressed. PmpB in males and CT157 (the most likely band C candidate) in females were shown to be up-regulated to varying degrees in samples across the patient groups. We also demonstrated that no other chlamydial antigens are up-regulated during a persistent C. trachomatis infection. In conclusion, although combinations of bands A, B, C, D and M differentiate between male and female patient groups under normal chlamydial growth conditions, during IFN-γ-induced persistence, only bands C (CT157) and M (CT413 - PmpB) are up-regulated thus suggesting a potential role in chronic C. trachomatis infection.