The kinetics of solution degradation of the antibiotic streptozotocin have been studied polarographically, spectrophotometrically, and by bioassay. Equations have been obtained to characterize degradation as a function of pH, buffer, and temperature. Bioassay has been correlated with these others and correction procedures devised to account for their limitations. Stability in vitro and in vivo has been predicted. A mechanism is proposed that accounts for the available data and describes the weakly basic nature of streptozotocin and indicates the presence of a N-nitro-somethylurea function in the antibiotic.