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The double-isotopic labelling technique was used to identify comprehensively proteins involved in α-glucan catabolism in Klebsiella pneumoniae NCTC 9633. Cells were grown with either glycerol in the presence of H-leucine or with glycerol plus maltose in the presence of C-leucine. Each labelled culture was then fractionated into the main subcellular components, i.e. the cytoplasm, periplasm, cytoplasmic and outer membrane. Corresponding fractions derived from H-labelled and C-labelled cells were combined, and the proteins were analyzed by polyacrylamide gel electrophoresis under denaturing conditions. Gel slices were then counted for H- and C-radioactivity, a positive deviation from the standard C/H ratio being evidence for the presence of a protein specifically induced by maltose in the culture medium. The protein pattern thus obtained was compared with the properties of proteins comprising a similar pathway for maltodextrin utilization in Escherichia coli K-12. Ample information which has been obtained mainly by genetic analysis is available about maltodextrin-utilizing enzymes in E. coli K-12. [ABSTRACT FROM AUTHOR]