For practical purposes the casein content of milk is often estimated from the crude protein content using a general conversion factor. This is done even though it is known that several factors can influence the percentage of casein nitrogen in the total nitrogen (casein number). The uncontrolled variation in casein number affects optimization of cheese production because casein is a limiting factor for cheese yield. Fourier transform infrared spectrometry has recently been introduced for casein determination. Using this technique we obtained a standard error of prediction (SEP) of 0·033% for casein concentrations in the range 2·1–4·0%. The true prediction error was estimated as 0·025%. The correlation between casein numbers obtained by reference analysis and infrared spectrometry could be expressed by an R2 of 0·73 and an SEP of 0·89 for casein numbers in the range 70·7–81·0.