ABSTRACT: We have previously shown that co-incubation of Chlamydia trachomatis lipopolysaccharide (LPS) leads to premature sperm death by an apoptosis-like mechanism. It was always assumed that lipid A is the toxic component of LPS. Here we investigate the possible involvement of 3-deoxy-d-manno-octulosonic acid (Kdo), which is an additional component of the LPS in C. trachomatis. Highly motile preparations of sperm from normozoospermic patients were incubated for 6 hours with commercial sources of lipid A and Kdo. Conventional lipid A inhibitors, polymyxin B (PMB) and anti-CD14 monoclonal antibody (mAb) were used to test the ability of both lipid A and Kdo to induce an apoptotic-like response in mature sperm. Flow cytometry was used to determine apoptosis by the expression of annexin V. Caspase activity was also measured by fluorometry and by the use of a pan-caspase inhibitor and caspase-3 inhibitor. Both lipid A and Kdo at 50 μg/mL caused significant mortality of sperm. However, although PMB and anti-CD14 mAb were inhibitory to the activity of lipid A on sperm, no such effect was seen against Kdo. In the presence of either lipid A or Kdo, sperm death was caused by an apoptotic-like effect that was caspase mediated. We conclude that Kdo shares its spermicidal properties with lipid A and seems to kill sperm in a similar manner. These results provide an explanation for higher than expected levels of spermicidal activity of LPS that are not caused by lipid A.