A 39 kilodalton glycoprotein has been isolated from bovine mammary secretions by heparin-agarose affinity chromatography and gel filtration. It is a minor whey protein in mammary secretions collected during the nonlactating period, but is clearly detectable by affinity chromatographic and immunoblotting techniques. It is not detectable by these techniques in milk or colostrum. This protein is not immunologically related to milk proteins, serum proteins or cytoskeletal proteins. The N-terminal amino acid sequence (36 amino acids) is not similar to other known proteins. Isolating this novel 39 kilodalton protein provides a specific marker for mammary function during the nonlactating period.