Chlamydia trachomatis is a human pathogen of the genital tract and ocular epithelium. It is an obligate intracellular parasite with a unique biphasic development cycle. C.trachomatis infection is the most common bacterial sexually transmitted disease in industrialized nations. Its ability to cause chronic disease makes it a serious economic burden and health threat to developed and developing countries. Although treatable, approximately 70% of C.trachomatis infections are asymptomatic, potentially leading to the development of chronic sequelae. Furthermore, chlamydial genital tract infection has been associated with an increased risk of cervical cancer and human immunodeficiency virus infection. Consequently, the development of an efficacious vaccine is the most convenient, potentially reliable and cost effective option to control chlamydial infection and disease complications. Anti-chlamydial protective immunity is essentially mediated by a T helper, type 1 (Th1), response that is dependent upon the presentation of antigen via major histocompatibility (MHC) class II molecules. While antibody secreting cells are not critical components of the primary effector response, they have been shown to be important for clearance of re-infection. Thus an ideal vaccine would be one capable of inducing both a strong Th1 T cell response and a strong mucosal antibody response. Currently there are very few efficacious vaccine candidates that have been identified and characterized. More specifically, there is only a limited number of known T cell antigens processed and presented by the human leukocyte antigen (HLA) class II molecules. This type of antigen is going to be essential to the development of an efficacious chlamydial vaccine. In this study we have identified a number of unique vaccine candidates using a novel in silico approach. In an attempt to overcome HLA polymorphism the whole chlamydial genome was screened for proteins containing epitopes predicted to bind multiple HLA class II molecules (i.e. predicted ‘promiscuous’ T cell epitopes). A wide range of HLA class II molecules were used in this screen to identify vaccine antigens that could potentially offer broad and ethnically balanced population coverage. This analysis identified a number of novel targets and was validated by the identification of a known chlamydial T cell epitope. A selection of these target proteins was cloned, expressed and purified. Recombinant protein was screened against serum samples from patients with both acute and chronic chlamydial infections. Two novel targets, hypothetical protein CT425 and ribonucleotide reductase small chain protein (NrdB) were identified as being immunoreactive. The in vivo protective efficacy of NrdB was analyzed using a mouse model. CD4+ T cells were harvested from NrdB immunized mice and adoptively transferred to naïve mice, which were subsequently infected at the genital site. NrdB immunization was found to confer a CD4+ T cell driven degree of protection similar to that seen with CD4+ T cells primed from a live challenge. The adjuvants and route of immunization used ensured immunological responses were initiated at both the systemic and local sites of infection. Immunization elicited a predominant Th1 response with primed T cells producing high levels of interferon gamma, an essential requirement for the development of an efficacious chlamydial vaccine. Furthermore, high titres of antigen specific IgG and IgA were produced following immunization, with sera derived antibodies demonstrating neutralization properties. NrdB is a highly conserved chlamydial protein with an essential role in the replication of chlamydiae and could play a central role in a multi-subunit vaccine against chlamydial genital tract infections.