Milk and milk concentrates containing 12-35% total solids were stored at 0, -2, -4, -6, -8, -12, and -20°C and protein stability of the thawed products was evaluated periodically. Samples stored at -4 to -12'C exhibited poorer protein stability than samples stored at higher or lower temperatures. Ultrafiltered (UF) skimmilk with permeate: retentate ratios of 10:90, 20:80, and 30:70 were stored at -8°C and they remained stable at least three times longer than frozen control samples of UF skimmilk stored at the same temperature. When the extent of UF was increased to 40:60, protein stability in the frozen retentate declined somewhat as compared to that of less concentrated retentates.