The amylopullulanse produced by Bacillus sp. DSM 405 was purified to homogeneity. It exhibited dual activity, cleaving the alpha 1-4 bonds in starch, releasing a range of malto-oligosaccharides, and also cleaving the alpha 1-6 bonds in pullulan, releasing maltotriose as the sole end-product. The enzyme was a glycoprotein and had a relative molecular mass of 126 000 and an isoelectric point of 4.3. While the enzyme was optimally active on starch at pH 6.5 and at pH 6.0 on pullulan, activity on both substrates was maximal at 70 degrees C. Kinetic analyses of the enzyme in a system that contained both starch and pullulan as two competing substrates demonstrated the dual specificity of the enzyme. Chemical modification of the carboxyl groups within the active centre of the protein showed that one active site was responsible for hydrolysis of the alpha 1-4 and alpha 1-6 bonds in starch and pullulan respectively. This is the first comprehensive investigation of an amylopullulanse produced by an aerobic bacterium, showing a single active site responsible for both activities.