Interaction of cysteine with type I-collagen from bovine achilles tendon in the aqueous solutions has been examined using square wave voltammetry (SWV) and cyclic voltammetry (CV) techniques. In the absence of cysteine, type I-collagen gives a reversible peak at -0.114 V in Britton-Robinson (B-R) buffer (pH 4.0). The electrochemical parameters (I(p)/f, E(p)/f, E(p)/pH, I(p)/pH, I(p)/v, I(p)/v(1/2)) of type I-collagen have been also studied. In addition, it has been determined that there is a linear relationship between current and concentration of type I-collagen. On the other hand, cysteine exhibits a reversible peak at -0.383 V due to the reduction of mercurous cysteine thiolate. By using a hanging mercury drop electrode in aqueous solutions, SWV and CV voltammograms obtained for type I-collagen in the presence of cysteine indicate that there is an interaction between type I-collagen and cysteine. In the presence of cysteine, peak current of type I-collagen decreases and a new peak is observed at -0.121 V for cysteine which is bonded to type I-collagen.