The concentrations of tumor-associated trypsin inhibitor (TATI) in the seminal plasma of infertile males was studied. The TATI levels in seminal plasma were not correlated with either sperm count or ejaculate volume. High levels were observed in some men with unexplained infertility and high or normal sperm counts, whereas normal levels were observed in males with antisperm antibodies. The concentrations in seminal plasma were stable in the same subjects. These results suggest that TATI may be an important marker of reproductive pathology in men.