The amino acid composition, essential amino acid index (EAA-I), pepsin-pancreatin in vitro digestibility and pepsin-pancreatin-digest-residue-index (PPDR-I) of the rumen bacterial and protozoal protein of cows fed urea and ammonium salts as their sole source of nitrogen (0-cows) or as a partial source (ULP-cows), and of cows on normal protein-rich feed (NorP-cows), were determined. The amino acid compositions of the rumen bacteria showed very slight changes even though the diets were very different. The amino acid compositions of the pure bacterial strains isolated from the rumina differed slightly. The amino acid compositions of the rumen protozoa of the ULP- and NorP-cows differed only with respect to isoleucine and tyrosine. Protozoa could be found only occasionally in the rumen of the 0-cow, there being only two species. Their nutritional significance was obviously very small. When the nutritional quality of the microbial protein was studied on the basis of its amino acid composition it was found that the EAA-I of bacteria did not differ significantly. Neither did the EAA-I of protozoa differ. The pepsin-pancreatin in vitro digestibility of protozoa was higher on all the feeds than that of bacteria. The rumen bacterial in vitro digestibility with 0-cows differed from that of the ULP-samples but not from that of the NorP-samples. The digestibility of single amino acids, With the exception of diaminopimelic acid, glycine and alanine, did not differ from the digestibility of the total amino acids. The much larger number of bacteria in the rumen of 0-cows compared with those of ULP- and NorP-cows compensates in this way for the lower digestibility of bacterial protein in 0-cows. The PPDR-I of both bacteria and protozoa were well correlated with the in vitro digestibility.