A reference milk system (Clarke system) using primary reference milks made from preserved natural bovine milks (PNM) and standard reconstituted milk (SRM) has been developed to enable accurate calibration of milk analysing instruments. The PNM have values of 2, 3, 4, 5 and 6% fat and 3·5, 2·7, 3·4, 4·2 and 3·3% protein respectively and can be stored for 4 months without detectable change in fat or protein content. The above reference milk system utilizes SRM as a secondary reference milk to enable regular checking of the stability of the instrument calibration during routine testing (e.g. after every 100 samples). Over several months seven laboratories using 13 fat-testing instruments and seven protein-testing instruments achieved high levels of accuracy in weekly calibrations (coefficient of variation 1·1%) when they used the reference milk system and adhered to the prescribed calibration criteria.