Summary. Lactoferrin isolated from sow milk (about 0\m=.\6mg/ml) was shown to be chromatographically homogeneous, an observation supported by electrophoresis and by reaction against monospecific anti-lactoferrin antiserum. Isoelectric focusing showed multiple forms of the protein (i.e.p., 9\m=.\3to 10\m=.\0)converted by neuraminidase to one form (i.e.p., 9\m=.\65).Boar seminal plasma contains immunologically identical lactoferrin (0\m=.\1to 0\m=.\5mg/ml) which binds strongly to boar spermatozoa. Lactoferrins are a group of iron-binding proteins (Groves, 1960; Johansson, 1960) found widely in mammalian external secretions (Masson, Heremans & Dive, 1966). The concentrations observed show marked variations between species (Masson, 1970). The protein usually occurs in an almost iron-free form. There have been earlier reports of the distribution of lactoferrin in the reproductive tract secretions of a number of animals (Hekman & Rümke, 1968; Roberts & Boettcher, 1971). This report concerns the isolation, by a modification of the method used for human lactoferrin by Querinjean, Masson & Heremans (1971), and characterization of lactoferrin from sows' milk. The study also concerns the occurrence of lactoferrin in boar seminal plasma. Whole milk, collected from at least ten lactating sows, was pooled and frozen until use. The milk (700 ml) was defatted by centrifugation (10,000 g for 20 min), filtered through a coarse gauze and adjusted to pH 7 with 400 mil- Na2HP04. Ammonium ferrous sulphate (10 mg) was added to the product (650 ml) to saturate the lactoferrin with iron. The mixture was stirred for 2 hr, diluted with an equal volume of buffer, 20 mM-Na2HP04, 150 mM-NaCl, pH 7, and the conductivity was adjusted to that of the diluting buffer with 4 M-NaCl. After adding 3 g CM-Sephadex (Pharmacia, G.B.), the mixture was stirred for a further 4 hr, allowed to stand and the supernatant fluid was then decanted. The CM-Sephadex containing the red iron-lactoferrin complex (Johansson, 1960) was washed twice, each time with 1·1 ml buffer, and was then used to form a column (2-5 25 cm). This was eluted with the 150 mM-NaCl-phosphate buffer until protein was no longer detected in the washings at 280 nm (Uvicord,