In 1939, Sorensen & Sorensen discovered a red protein in bovine milk. This component, now usually called lactoferrin, is a glycoprotein with a \g=g\-electrophoretic mobility, and consists of a single polypeptide chain with a molecular weight of 77,000. Like serum transferrin, it binds two ferric ions, but it differs in its amino acid composition, immunological properties and in the higher stability of its iron-complex at acid pH. These features apply to bovine as well as to human lactoferrin (Groves, 1960; Johansson, 1960; Montreuil, Tonnelat & Mullet, 1960; Blanc & Isliker, 1961; Masson & Heremans, 1968; Castellino, Fish & Mann, 1970; Querinjean, Masson & Heremans, 1971). In the human, lactoferrin is not only found in milk but also occurs in other external secretions. It is synthesized by three types of tissues: the acini of certain glandular epithelia such as those from the bronchial mucosa and the salivary, lachrymal and mammary glands; the tubules of the kidney; and those myeloid cells which give rise to neutrophil leucocytes (Masson & Heremans, 1966; Masson, Heremans, Prignot & Wauters, 1966; Masson, Heremans, Schonne C Masson, Heremans & Schonne, 1969). Bovine lactoferrin has been shown to occur in the milk (Morrison & Allen, 1966) and the secretions from Harderian and lachrymal glands, but data are lacking on its presence or absence in other fluids. In other mammals, only milk and extracts of leucocytes have been studied with respect to their lactoferrin content (Masson et ., 1969; Baggiolini, de Duve, Masson & Heremans, 1970; Masson & Heremans, 1971). When a pool of bovine oestrous cervical mucus was filtered on Sepharose 6B, equilibrated with 0-1 M-tris buffer (pH 8) containing 2 M-NaCl, it resolved into two peaks. The first peak was eluted with the void volume and corresponded chiefly to sparingly soluble mucins. The second peak, after concentration by ultrafiltration, had a pinkish hue. Electrophoresis on cellulose acetate in barbitone buffer (pH 8-6) showed three distinct bands and one diffuse zone, when stained with Ponceau S (PI. 1, Fig. 1). The leading band migrated as albumin and according to immunoelectrophoresis, the diffuse zone correspon¬ ded to immunoglobulins. The slowest band could have been lysozyme as its cathodal mobility was identical to that of the corresponding enzyme in the human. The major band of y-mobility was an iron-binding protein. After addition of [59Fe]citrate, this band showed up on the X-ray film exposed to the