The K5 capsular polysaccharide antigen of some Escherichia coli strains is the non-sulphated precursor in heparin biosynthesis. It is composed by two components, 16 000 and 1500 Da, whose ratio depends on the activity of the extracellular form of a lyase synthesized by the same K5 producer strain. The lyase activity on the K5 chain size was greatly influenced by the medium composition employed for the lyase production. The control of lyase activity results in defined ratios of the two components in the K5 polysaccharide that is suitable for the semisynthetic production of heparin-like molecules.