Seminal plasma transferrin levels as well as serum FSH and LH levels were measured in fertile (normozoospermic) and infertile (oligozoospermic and azoospermic) men in an effort to find out whether transferrin levels correlated with sperm density and serum FSH and LH levels. Seminal plasma transferrin levels were found to be significantly lower (p < 0.0005) in the infertile group than the fertile group. Seminal plasma transferrin levels can be regarded as a functional parameter of Sertoli cell function in men.