The activity expressed by lactate dehydrogenase in the human erythrocyte has been compared with the activity displayed by the isolated enzyme in vitro. Enzyme activity was measured by using 1H spin-echo NMR to measure non-invasively the velocity of hydrogen label exchange between the C-2 positions of two methyl-labelled lactate species. This method has significant advantages over a method which has been described previously. The exchange velocity observed in the cell was much lower than that expected based on a comparison with measurements of the exchange velocity in vitro under conditions thought to simulate the intracellular environment. Measurements of enzyme inhibition in cell extracts suggest that the low intracellular activity is due to inhibition of the enzyme by low-molecular-mass compounds present in the cell.