Transferrin and ceruloplasmin have been measured by a solid-phase chemiluminescent method in seminal fluid and circulating blood of normal and vasectomized subjects (1 year after operation). This study has confirmed that approximately 80% of seminal transferrin comes from the testis, while seminal ceruloplasmin was not found different in the two groups. In patients affected by azoospermia due to seminiferous tubular damage (n = 15) in whom an obstruction was previously excluded, seminal transferrin was always below the normal range. On the contrary, seminal ceruloplasmin was always in the normal range, and circulating follicle-stimulating hormone was found above the normal range only in nine cases. No correlation was found between seminal transferrin and circulating follicle-stimulating hormone in such groups. In an unselected group of infertile patients with decreased sperm concentration and/or sperm motility, seminal transferrin was found correlated with the sperm count. These studies seem to suggest that seminal transferrin is a reliable index of seminiferous tubular function.