The concentration of transferrin in seminal plasma and serum levels of LH, FSH, testosterone, and prolactin were measured in 94 male patients attending our infertility clinic. The concentration of transferrin in patients with azoospermia was lowest among azoospermia, oligozoospermia, and normozoospermia groups. There was significant positive correlation between the concentration of transferrin in seminal plasma and sperm density. However, seminal plasma transferrin was inversely correlated with serum levels of LH, FSH, and prolactin, respectively. It is suggested that transferrin in seminal plasma contributes to the activation of seminiferous tubules and can be used as a reliable index of Sertoli cell function.