ABSTRACT. Lactoferrin purified from canine seminal plasma by a three-step chromatography procedure had a molecular mass of 75.2 kDa and cross-reacted with antiserum to equine seminal plasma lactoferrin. Seminal plasma lactoferrin concentrations were determined by a competitive enzyme-linked immunosorbent assay (ELISA) by using rabbit anti-equine lactoferrin antibody and alkaline phosphataselabeled goat anti-rabbit IgG antibody in 14 normal dogs and found to range from 12 to 197 µg/ml, with a mean value of 77 ± 59 µg/ml (the mean ± SD). Seminal plasma transferrin concentrations were determined by a sandwich ELISA with goat antibody to canine serum transferrin and alkaline phosphatase-conjugated goat anti-canine transferrin antibody and found to range from 0.32 to 12.6 µg/ml, with a mean value of 2.44 ± 3.25 µg/ml. The lactoferrin concentration significantly correlated with the sperm concentration (r=0.7025, P<0.01), but there was no significant correlation between the seminal plasma transferrin concentration and sperm density. These results indicate that seminal plasma lactoferrin, but not transferrin, reflects gonadal function. KEY WORDS: canine, lactoferrin, seminal plasma, sperm, transferrin.J. Vet. Med. Sci. 65(6): 679-684, 2003 Lactoferrin is an iron-binding protein (molecular mass of 80 kDa) that is found in various exocrine fluids, blood plasma and specific granules of neutrophils [15]. Lactoferrin has been identified in the seminal plasma of humans [10,19], swine [21] and horses [14]. Lactoferrin seems to be synthesized mainly in the epididymis of mice [27], swine [16] and horses [7], and it is present in the prostate and seminal vesicles, but not in the epididymis, of humans [26]. The physiological role of lactoferrin in the male reproductive tract remains unclear, although lactoferrin has been identified as the sperm-coating antigen in humans [10] and swine [16,21].Transferrin is homologous to lactoferrin and is a major iron transport protein in the circulation [1], and is also found to be present in the seminal plasma of humans [12] and cattle [9]. Transferrin is secreted by the Sertoli cells [13,22] and plays an important role in spermatogenesis by providing iron to developing germ cells, which have specific transferrin receptors [11,24]. Seminal plasma transferrin is known to be a good index of gonadal function in humans [2, 6] and cattle [9], although 20% and 40% of transferrin in semen is contributed by the accessory sex glands of humans [12] and cattle [9], respectively.No information is available about either lactoferrin or transferrin in canine semen. In the present study, an enzyme immunoassay for measuring the concentrations of lactoferrin and transferrin in canine seminal plasma was developed and the relationships between seminal plasma lactoferrin and transferrin concentrations, the number of sperm and sperm motility were examined. MATERIALS AND METHODSSeminal samples: Ejaculate was obtained from 14 normal mature dogs (age unknown). Seminal properties including sperm number and sperm moti...