ABSTRACT. Total 78 semen samples were obtained from 27 Thoroughbred stallions (aged 6 to 27 years), and were subjected to quantification of lactoferrin (Lf) in seminal plasma and examination of the seminal properties. The seminal plasma Lf concentration varied from 21 to 689 µg/ml, with a mean value of 244 ± 151 µg/ml (S.D.). The seminal plasma Lf concentration and total seminal plasma Lf positively correlated with the sperm concentration (r=0.5938, P<0.001) and with the total sperm number (r=0.6959, P<0.001), respectively. There was no correlation between seminal plasma Lf and sperm motility. These results suggest that seminal plasma Lf reflects gonadal function. KEY WORDS: equine, lactoferrin, seminal plasma.J. Vet. Med. Sci. 65(11): 1273-1274, 2003 Lactoferrin (Lf) is a transferrin-like protein consisting of two lobes, each of which binds one ferric ion [2]. Lf occurs in various exocrine fluids, blood plasma and specific granules of neutrophils [7,13]. Lf displays bacteriostatic activity by chelating iron necessary for microbial growth [13], and prevents the formation of hydroxyl radicals [11]. Lf found in substantial amounts in milk is considered to promote iron absorption in infants [7]. Lf has been found in the seminal plasma of various mammalian species [4,6,10,[13][14][15], and identified as the sperm-coating antigen [4,8,14]. It is known that Lf is synthesized in the epididymis in various mammalian species including horses [3], pigs [8], and mice [17]. From these findings, it may be considered that Lf is involved in sperm maturation in the epididymis. Recently, we found the positive correlation between the seminal plasma Lf concentration and the sperm density in dogs [10], suggesting that Lf secretion relates to gonadal function. The present study was performed to determine whether there is also this relationship in mammals other than dogs.Total 78 semen samples were obtained from 27 normal Thoroughbred horses (aged 6 to 27 years) in January and February. Seminal properties including the sperm number and sperm motility were immediately examined as described [9] and the semen samples were subsequently centrifuged at 1,650 × g for 20 min at room temperature. The seminal plasma was collected and stored at -25°C until use.The concentration of Lf in equine seminal plasma was measured by a sandwich enzyme-linked immunosorbent assay that was recently developed in our laboratories [6]. Total seminal plasma Lf was expediently calculated by multiplying the seminal plasma Lf concentration by ejaculate volume, because the spermatocrit value was not measured. Protein was measured according to the method of Lowry et al. [12] with bovine serum albumin as the protein standard. Correlation coefficients were determined by simple linear regression.The seminal plasma Lf concentration varied from 21 to 689 µg/ml, with a mean value of 244 ± 151 µg/ml (S.D.). Figures 1A and 1B show a positive correlation between the seminal plasma Lf concentration and sperm concentration (r=0.5938, P<0.001) and between total seminal plasma Lf...