A particular relation exists between casein micelle and vitamin A. In vitro, large amounts of retinol are fixed to acid casein and its different fractions by hydrophobic bindings. The binding on the hydrophobic amino-acid residues (Trp, Phe) is greatly facilitated by a configurational change in the molecule exposed to physico-chemical parameters: alcalinity and heat treatments. Another amount of retinol is even more strongly fixed by a binding which can only be broken by saponification. Casein plays an important role in stabilizing retinol which does not degrade over time or during heat treatments. In vivo, when the retinol availability is measured by the vitamin A content in the liver, acid casein somewhat increases the retinol efficiency and, even more so, the retinal efficiency. Nevertheless, this favourable action is only observed under particular conditions: balanced diet, casein from milk, etc. For the moment, the animal data cannot be directly linked with the properties of binding appearing in vitro.