A submerged batch cultivation of Moraxella bovis in a medium containing enzymic casein hydrolyzate and supplemented with dialyzed ram blood was described. Up to the growth limitation the bacteria grew exponentially without a lag phase and with a doubling period of 64 min. During the exponential phase no significant decrease of viable cells and cell autolysis were observed. Amino acids were the limiting factor. At the end of growth glutamic acid, threonine and serine were detected at the lowest concentrations. Substrate limitation led to an irreversible decrease of the metabolic activity. Growth yield on oxygen was 2.3 X 10(9) cells per mg O2. The fraction of respiration required for growth-uncoupled processes was negligible. During the cultivation ammonia was produced and the pH increased but it was not the inhibitory factor. Respiration was not limited by oxygen at concentrations higher than 0.63 mumol O2 per L. Sufficient pili were produced.