Three types of potato starch have been hydrolyzed by an alpha-amylase from Bacillus subtilis. The courses of enzymatic degradation were observed with gel-permeation chromatography. The changes in molar mass distributions by enzymatic attack alter (depending on the type of starch) the enzyme concentration and incubation time. Additionally, macromolecules of native potato starch are destroyed by mechanical treatment and acid hydrolysis. Initial distributions of molar mass for partly soluble and native potato starch are bimodal from the interpretation of the clustered structure of amylopectin. The kinetics of the reaction are described with the Michaelis-Menten rate equation with the specific activity for native potato, partly soluble, and Zulkowski starch of 3,000; 2,900; and 2,630 mu mol mg(-1) protein min(-1), respectively. The Michaelis-Menten parameters are 1.4, 1.9, and 6.0 g l(-1), respectively. The significantly higher K-m value of Zulkowski starch is explained by the low degree of polymerization and the content of alpha-1,6 branching bonds.