The levels of transferrin in seminal plasma of a large sample of infertile men (n = 287) were measured by radioimmunoassay. A group of recently pregnancy-proven fertile men (n = 20) was used as control and a small group (n = 6) of vasectomized men was studied to determine the origin of seminal transferrin. Infertile men had lower transferrin values than fertile men, although the difference was not significant in the case of normozoospermic infertile men (0,05 less than p less than 0,1); in the case of oligozoospermic infertile men the difference was highly significant (p less than 0,001). The values of transferrin in severely oligozoospermic, azoospermic and vasectomized subjects suggested that 60% of seminal transferrin could be of testicular origin. In infertile patients transferrin fall along with sperm count; there is a strong correlation between transferrin and sperm density. FSH levels were measured in a group of severely oligozoospermic and azoospermic patients (n = 41); although the patients with elevated FSH had lower transferrin levels than the patients with FSH within the normal range, the difference was not significant. The results of this study show that transferrin could be a useful marker of seminiferous tubular function but more work is needed to assess its relevance for clinical practice.