Time-dependent changes in the intracellular Na+/K+ ratio of Escherichia coli, induced by the nitrofuran derivative HN32 [2,4-diamino-6-(5-nitrofuryl-2)-5-ethylpyrimidine], were measured by laser-induced mass spectrometry of single bacterial cells. The results show good agreement with data on viable cell and total cell counts, release of ATP and 14CO2 production demonstrating that the single cell analysis of intracellular sodium and potassium concentrations may supply reliable information on cell viability and, furthermore, offer additional information not available from established gross methods.