The interaction of novobiocin (NOV), an aminocoumarin antibiotic, with cysteine was studied by square-wave voltammetry technique on the hanging mercury drop electrode in different pH values. After the addition of NOV into the cysteine solution, the peak current of mercurous cysteine thiolate decreased and its voltammetric peak potential shifter to more positive values. Voltammetric results showed that NOV binds with cysteine forming 1:1 nonelectroactive molecular complex by means of electrostatic and hydrogen-bonding interactions. The binding constants of NOV with cysteine at pHs 5.7 and 10 were calculated to be 3.06 x 10(3), 1.54 x 10(4) and 1.06 x 10(5) M(-1), respectively. Furthermore, a possible mechanism of such interaction was also discussed.