In order to synthesize bifunctional antitumor compounds, the interaction of streptonigrin with [AuCl4]- has been studied. Using absorption, circular dichroism, and fluorescence measurements, we have shown that streptonigrin forms with Au(III) a 1:1 Au(III)-streptonigrin complex. This complex is very stable as long as gold is in the trivalent state and is able to inhibit glutathione reductase activity. In the presence of biological agents such as NADH and reduced glutathione, Au(III) is slowly reduced to Au(I) and removed from its binding site to streptonigrin. Original streptonigrin is thus recovered. This complex exhibits antitumor activity against P-388 leukemia which compares with that of the free drug.