The chemical stability of the novel anticancer agent carzelesin in aqueous buffer/acetonitrile (1:1, v/v) mixtures has been investigated utilizing a stability-indicating reversed-phase high-performance liquid chromatographic assay. The degradation kinetics of carzelesin has been studied as a function of pH, buffer composition, ionic strength, and temperature. Degradation of carzelesin follows (pseudo-) first-order kinetics. A pH-rate profile, using rate constants extrapolated to zero buffer concentration, was constructed demonstrating that carzelesin is most stable in the pH region 1-4. The degradation rate of carzelesin was not significantly affected by buffer components and by the ionic strength. In addition to the formation of the degradation products U-76,073, U-76,074, and aniline in alkaline medium and in acetate buffer solution, another degradation product was formed in acetate buffer solution. In perchloric acid buffer solution (pH* < 3), U-76,073 and U-76,074 could not be detected as degradation products.