The cationic hydroxyethylcellulose Polyquaternium 10 (PQ10) was found to produce a dose-dependent destabilization of casein micelles from whole or skim milk without affecting the stability of most of the whey proteins. The anionic phosphate residues on caseins were not determinant in the observed interaction since the destabilization was also observed with dephosphorylated caseins to the same extent. However, the precipitation process was completely inhibited by rising NaCl concentration, indicating an important role of electrostatic interactions. Furthermore, the addition of 150 mM NaCl solubilized preformed PQ10-casein complexes, rendering a stable casein suspension without a disruption of the internal micellar structure as determined by dynamic light scattering. This casein preparation was found to contain most of the Ca2+ and only 10% of the lactose originally present in milk and remained as a stable suspension for at least 4 months at 4 degrees C. The final concentration of PQ10 determined both the size of the casein-polymer aggregates and the amount of milkfat that coprecipitates. The presence of PQ10 in the aggregates did not inhibit the activity of rennet or gastrointestinal proteases and lipases, nor did it affect the growth of several fermentative bacteria. The cationic cellulose PQ10 may cause a reversible electrostatic precipitation of casein micelles without disrupting their internal structure. The reversibility of the interaction described opens the possibility of using this cationic polysaccharide to concentrate and resuspend casein micelles from whole or skim milk in the production of new fiber-enriched lactose-reduced calcium-caseinate dairy products.