The content of casein in milk is determined by two measurements of infrared absorbance in a milk sample by infrared spectrometry before and after a separation of the casein. The casein content is calculated by use of absorbance data recorded during the two absorbance measurements. The new method is considerable faster than the known wet-chemical methods, such as the normal wet chemical reference method for casein determination in milk using a Kjeldahl nitrogen determination of the milk sample, then a coagulation of the milk, and finally a Kjeldahl nitrogen determination of the filtrate. Further the new method provides a more reliable accuracy than the know determination using a single infrared analysis of a milk sample.