A cows' milk identification test (COMIT) has been successfully developed for the detection of defined amounts of cows' milk (3–100%) in ewes' milk. The assay uses polyclonal antibodies raised in goats against bovine whey proteins. The test, an agar-gel immunodiffusion technique, uses agar plates with a printed template for correct placement of the test components (antisera discs, positive and negative milk reference discs and sample discs). The presence of cows' milk in the sample was recorded as positive when the precipitin lines appearing between the sample disc and the antisera disc fuse completely with those from the positive cows' milk reference disc. This test is reliable, practical and easy to perform in the field and in dairy factories. Furthermore, the nature and stability of the components of the test make it suitable for commercial development into kits which should be highly practical in any kind of inspection programme concerned with milk species identification.