Control of milk secretion

An isolated protein in glycosylated or unglycosylated form which inhibits milk secretion by lactating cows and which is present in the eighth significant peak, shown on the drawing as "6B", when a fraction of the whey proteins of the milk separated by ultrafiltration using filters of cut-off values 10 KDa and 30 KDa, is resolved on an anion exchange column of particles of monodisperse hydrophilic polymer having pendant --CH.sub.2 N(CH.sub.3).sub.3.sup.+ groups, the particle diameter being 10.+-.0.5.mu.m, using 10 mM imidazole buffer, pH 7.0 and a sodium chloride elution gradient of 0 to 1.0M and said protein having a molecular weight, as determined by gel filtration chromatography, of about 6.5 KDa, and an isoelectric point determined by isoelectric focussing of material from said peak 6B in a polyacrylamide gel on the product of the 6B peak within the range 5.1 to 5.4.