NMR relaxation measurements can provide a simple means for understanding biological activity of cells in solution with known composition. It has the advantage that it is an in situ, non-intrusive technique, and the acquisition is fast. The iron oxidation ability of Acidithiobacillus ferrooxidans was investigated using NMR relaxation measurements. The transversal relaxation is characterized by a time constant, T(2), which is sensitive to the chemical environment. Fe(3+) ion has more significant T(2) shortening than Fe(2+) ion. In the presence of A. ferrooxidans in solutions containing Fe(2+) ion, T(2) shortening was found with increasing time as the bacteria oxidize Fe(2+) to Fe(3+) ions. In the optimal growth medium, the bacteria concentration increased 80 times and high iron oxidation rate was found. In 10 mM K(2)SO(4) medium, however, bacteria concentration remained almost unchanged and the iron oxidation rate was significantly lower. (C) 2011 Elsevier Ltd. All rights reserved.