The interaction of ciprofloxacin with DNA was studied by using an electrochemical DNA biosensor. The binding mechanism of ciprofloxacin was elucidated by using constant current potentiometry and differential pulse voltammetry at DNA-modified glassy carbon electrode. The decrease in the guanine oxidation peak area or peak current at +0.9 V was used as an indicator for the interaction mechanism in 0.2M acetate buffer (pH 5). The binding constant (K) values obtained were 1.33+/-0.02 x 10(4) and 1.32+/-0.08 x 10(4) M(-1) with constant current potentiometry and differential pulse voltammetry, respectively. A linear dependence of the guanine peak area or peak currents was observed in the range of 40-80 microM ciprofloxacin, with a detection limit of 24 microM with r=0.995 and 9 microM with r=0.999 by using constant current potentiometry and differential pulse voltammetry, respectively. Moreover, the influence of sodium and calcium ions was also studied to elucidate the mechanism of ciprofloxacin-DNA interaction at different solution conditions, and this proved to be helpful in understanding the ciprofloxacin-DNA interaction.