Analysis of the concentrations of free amino acids in milk during growth of Lactococcus lactis subsp. lactis revealed a biphasic pattern of change during the logarithmic phase. During the first period there was little overall change in the total concentration of amino acids in the medium. The second phase was characterized by increased net liberation of free amino acids. There were also qualitative differences in the amino acids that were taken up and utilized during each period. The concentrations of Val, Leu and Ile decreased only during the early phase, while those of Ser, Arg, Thr and Met decreased only during the second phase. Gly and Ala were utilized throughout logarithmic growth. Gly uptake appeared to be greater during the second period and accounted for the largest proportion of free amino acid utilization at this time. It is possible that the biphasic nature of amino acid nutrition was due to increased consumption in late log phase of peptides derived from milk proteins by proteolysis. Increased activity of the arginine deiminase pathway during late log phase was inferred from increased utilization of Arg and liberation of citrulline and ornithine.