This International Standard describes the operating conditions for instruments used for the determination of fat, protein and lactose content of ex-farm milk, based upon the measurement of the absorption of mid-infrared radiation at wavelengths which are representative of each component analysed. NOTE 1 In practice, these measurements are made using commercial automatic or semi-automatic instruments defined in clause 5 and called "infrared instruments" in this International Standard. Any model of instrument which does not comply with the principle of analysis given in this International Standard or which incorporates modifications that may alter the principal characteristics of the apparatus (repeatability, accuracy, conditions of use), as well as the means of adjusting the calibration, will require a separate specific standard. NOTE 2 Not all instruments allow determination of the lactose content. Moreover, some instruments allow direct measurement of the water content. The total solids content can be estimated by adding together the fat, protein and lactose contents, a constant being used to correct for the salt content. The method described is applicable to the determination of the fat, protein and, as appropriate, lactose content of ex-farm milk. The method is also applicable to the analysis of milk of other species (goat, ewe, buffalo, etc.) and processed milk provided a specific calibration of the instrument (see clause 7) is made.