Chlamydia trachomatis is an obligate intracellular bacterium which causes the most prevalent sexually transmitted bacterial infection throughout the world. According to the World Health Organization, at least 90 million chlamydial infections are detected annually worldwide. While the role of C. trachomatis in female infertility is well established, data revealing that this organism cause male infertility are still controversial. In this study the effect of "home-made" C. trachomatis LPS and two LPS fractions, lipid A and Kdo on human spermatozoa were investigated in detail. The effect of heated C. trachomatis elementary bodies (EBs) on the viability of spermatozoa and the potency of C. trachomatis LPS in comparison with three LPSs from the Enterobacteriaceae family were also studied. Treatment of spermatozoa with heated-EBs caused a significant increase in sperm death rate and revealed that the toxic component of EB was LPS which is a heat-stable complex. Incubation of spermatozoa with chlamydial LPS showed that this LPS at a concentration of 0.1 pg/m1 induced significant death of spermatozoa, whereas LPSs from E. coli, Kpneumoniae, and S. marcescens, did so only at 50 µg/ml. The effect of the two main LPS fractions of C. trachomatis, lipid A and Kdo, on spermatozoa was also investigated. A small amount of lipid A was isolated from chlamydial LPS and Kdo was purchased. Spermatozoa were treated with these fractions separately. Kdo was shown to be as spermicidal as lipid A, however, unlike lipid A, Kdo was not affected by anti-CD14 antibody or polymyxin B. In an attempt to clarify the mechanism of interaction of chlamydial LPS with spermatozoa and the role of the TLR pathway and TNF-a production, an experimental model using HeLa and THP-1 cell lines was prepared. Because of a lack of a sufficient amount of chlamydial lipid A, lipid A like Kdo was also obtained commercially. While chlamydial LPS and commercial lipid A were able to signal via TLR2 and/or TLR4 in transfected HeLa cells and to induce TNF-a production in THP-1 cells, Kdo failed either to signal through TLRs or stimulate TNF-a secretion. Finally, the role of chlamydial LPS, lipid A or Kdo in ROS production which could potentially induce sperm death, and the mechanism of sperm death induced by these fractions were also studied. In this investigation, spermatozoa were pre- incubated with five ROS scavengers; ascorbic acid, catalase, reduced glutathione, superoxide dismutase (SOD), and a-tocopherol, prior to treatment with the above- mentioned fractions. ROS scavengers used in this investigation neutralised the spermicidal activity of the fractions. It is suggested that these fractions possibly induce their pathogenic effect via ROS production. Results from an annexin V/PI binding assay and quantitation of caspase-3 activity in spermatozoa stimulated with chlamydial LPS, Kdo, and lipid A revealed that LPS and its fractions induced sperm apoptosis which was primarily caspase-mediated. In a hypothetical model which is shown in chapter 6, we illustrate how chlamydial LPS exerts its spermicidal activity which is via CD14, possibly TLRs, ROS production and finally apoptosis induction.