Concentrations of tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) in seminal fluid, as well as levels of sperm lipid membrane peroxidation, were investigated in fertile and infertile men. Semen samples, obtained by masturbation from 37 infertile and 14 fertile men, were examined for the presence of TNF-alpha and IL-6. The level of lipid peroxidation of the sperm membrane was measured by determining malondialdehyde (MDA) formation. The correlation between the IL-6 and the TNF-alpha concentrations in seminal plasma with the levels of lipid peroxidation of the sperm membranes was statistically evaluated. The IL-6 concentration in seminal plasma of infertile men was significantly higher than that of fertile men (p < .05). Similarly, the level of membrane lipid peroxidation was higher for the semen of infertile men than that of fertile men (p < .001). A significant positive correlation was found between IL-6 levels in seminal plasma and membrane sperm lipid peroxidation (p < .002), but not between this parameter and TNF-alpha levels in seminal plasma. These findings suggest a possible association between IL-6 seminal plasma levels and lipid peroxidation of sperm membrane. Stimulation of reactive species production by human sperm and leucocytes, induced by the high levels of IL-6, could explain these results.