Human seminal plasma contains large amounts of transferrin, which is a protein secreted mostly by Sertoli cells. It has been suggested that the concentration of transferrin may serve as a possible clinical marker of Sertoli cell function. Therefore the concentration of this protein in human seminal plasma from fertile and infertile men has been evaluated in order to find a relationship between transferrin concentrations and human semen parameters and plasma FSH levels. Findings show that seminal transferrin in subjects with oligozoospermia or azoospermia is significantly lower than in controls, and that it is strongly related to sperm count. Results also indicate that transferrin secretion can be impaired when plasma FSH levels are still normal, suggesting that seminal transferrin is an early and specific marker of Sertoli cell function. These results, however, do not clarify whether impairment of transferrin secretion by Sertoli cells is due to an organic dysfunction or to an organic secretory alteration.