Heparins with different structures and physico-chemical properties were evaluated for their capacity to inhibit human leukocyte elastase activity in vitro by using a chromogenic substrate. Heparin from bovine intestinal mucosa and heparan sulfate from bovine spleen were extracted and purified, and their purity, structures, and physico-chemical properties were evaluated. Slow moving and fast moving heparin species were obtained by selective precipitation as barium salt, and partially desulfated and re-N-sulfated heparin was produced by chemical modifications. Heparins with different molecular mass (from 950 to 7820), narrow polydispersity and the same charge density were produced by a chemical depolymerization process in the presence of free radicals, and further gel-permeation chromatography. Heparins strongly inhibit elastase activity, and there is a significant linear dependence between charge density (sulfate-to-carboxyl ratio) and enzymatic activity. We also found a significant linear correlation between the percentage of N-sulfate groups and increased inhibition of elastase activity and between the percentage of iduronic acid and enzymatic activity. Heparin samples with a M(r) greater than about 2000-3000 inhibit the HLE activity to the same extent (about 59%) whilst two fractions with a M(r) of 1530 (29% inhibition of HLE activity) and 950 (4% inhibition of HLE activity) have less capacity to produce a decrease in the enzymatic activity.