Microbial protein produced in the rumen is not sufficient to meet the requirement for amino acids of the high producing dairy cow. The microbial protein thus need to be supplemented with a certain amount of undegraded protein which match the microbial protein both quantitatively and qualitatively to enable an efficient utilization for milk production. This paper summarizes findings with respect to amino acid composition and digestibility of microbial amino acids. The main emphasis is however on recent findings with respect to degradability and digestibility of individual amino acids in feed proteins. The results seem to indicate that degradability and digestibility estimates obtained for nitrogen may not always apply to individual amino acids. It is concluded that more research is needed to identify the ideal profile of the amino acids absorbed from the intestine of lactating cows. Results of such efforts are needed to compose an ideal protein supplement eventually by using protected amino acids to balance the microbial protein to meet the amino acid requirement of dairy cattle with a minimum of rumen undegraded protein.