The role of cell-mediated immunity in the etiopathogenesis of male infertility is far from being defined. The cytokine interleukin-2 (IL-2) has a key role in T-cell mediated immune responses. The aim of this study was to confirm the presence of IL-2 in human seminal plasma, to show eventual differences between IL-2 concentrations in fertile and infertile subjects, and to show potential relationship between IL-2 amounts in semen and spermiogram parameters. Forty-three subjects entered the study, 20 with proven fertility and normal semen quality (fertile group) and 23 with male infertility of at least 2 years and poor semen quality (infertile group). IL-2 levels of seminal plasma in infertile subjects (444.3 +/- 40.5 fmol/mL) were significantly higher than those in fertile subjects (251.3 +/- 42.7 fmol/mL). There was a significant negative correlation between IL-2 levels and sperm count, motility, and morphology. No correlation was found between IL-2 levels and leukocyte count. These findings confirm IL-2 to be in seminal plasma, show increased IL-2 secretion in the infertile group, demonstrate negative correlations of IL-2 levels with main spermiogram parameters, and indicate no correlation with leukocyte count. IL-2 concentration in seminal plasma may therefore be considered as a potential marker in male infertility.