The effect of protein : energy ratio’s in milk replacers based on whey powder as the main energy source was studied. In a digestion-balance trial 9 milk replacers containing decreasing levels of crude protein but increasing levels of high fat whey powder were fed to 45 Friesian bull calves in a set of 9 treatments. A protein rich calf starter concentrate (18%) was available ad libitum. Concentrate intake, nitrogen digestibility and nitrogen retention were significantly influenced by the protein content and the protein:energy ratio in the milk replacers. The percentage apparent N digestibility and the efficiency of N retention decreased linearly when less than 18% of the DE in the milk replacer was derived from protein. The apparent optimum protein content of 2l% as well as the optimum protein:energy ratio of 9,99 g DCP per MJ DE were achieved with a mixture containing 60% high fat whey powder and 40% skimmed milk powder.