Two heparin families (SM- and FM- heparins) have been fractionated with barium by selective precipitation at different temperatures. Fragments were obtained from these two fractions by enzymic and chemical degradation. Some structural features and pharmacological activities of FM- and SM-heparins as well as the fragments are now reported. The fragments, prepared by ascorbate/H2O2 oxidation (M.W. 4,500) or heparitinase II (M.W. 5,500), are enriched with trisulfated disaccharide units whereas heparin (M.W.12,500) and FM-heparin (M.W. 10,200) contain also N-sulfated and N-acetylated disaccharides. The chemical and enzymic fragments show 1/3 to 1/5 of the anticoagulant activity of heparin by the USP and APTT methods. The LPL releasing activity is also low in the fragments as well as the AXa activity measured by the chromogenic method. On the other hand the AXa activity measured by the Yin and Wessler procedure is two times higher in the fragments when compared to heparin. The fragments resulting from heparin after heparitinase II degradation and ascorbate/H2O2 oxidation were rich in trisulfated disaccharide units whereas heparin and FM-heparin contained also measurable amounts of monosulfated and N-acetylated disaccharide units. The mechanism of ascorbate oxidation and the structural requirements for the pharmacological activities of heparin is discussed in view of these and other findings.