Inappropriate milk-like secretion from the breasts is not infrequently encountered in patients. Usually these secretions have been identified as milk on the basis of their characteristic appearance, and only rarely has their identity as milk been proven by appropriate analysis. Since milk is chemically defined as a lactose-containing mammary secretion, the presence of lactose in a breast secretion identifies it as milk irrespective of its appearance or the presence of other constituents. Lactose can he readily identified by thin-layer chromatographic methods, which despite their inherent speed and sensitivity have not been widely utilized. A method using commercially prepared cellulose layers and a sandwich developing apparatus is presented in detail. This method utilizes current advances in chromatographic techniques and permits rapid and accurate identification of lactose in breast secretions. Such secretions from 10 nonpuerperal women were screened for the presence of lactose. The milk-like secretions from 8 contained varying amounts of lactose, which was not detectable in the secretions from the other 2. There was no correlation between the appearance of the secretion and the presence of lactose.