The acid extractable amino acid pool (AEP) demonstrates time and concentration dependent saturation kinetics in cold-incubated (0-2 degrees C) Escherichia coli cells. Attention is drawn in particular to the contribution of the AEP to the total non-covalently bound amino acid intracellular levels (total pool) at low exogenous amino acid concentrations. Amino acids compete with one another for associations which internally constitute the AEP. Although physicochemically related amino acids tend to be better competitors than other amino acids, the physicochemical relatedness was by no means the sole, or necessarily the main, factor determining competitiveness. On the return of cells to incubation temperatures allowing protein synthesis to proceed, the contribution of the AEP to protein synthesis was explored.