We recently reported that the subcutaneous (s.c.) administration of a low-molecular-weight heparin (LMWH) fraction significantly inhibited de novo angiogenesis in the mesentery induced by the intraperitoneal (i.p.) injection of saline to adult rats compared with unfractionated heparin and high-molecular-weight heparin (HMWH) fractions. The present study assesses the effect on basic fibroblast growth factor (bFGF)-mediated de novo angiogenesis in the mesentery of the systemic administration of a LMWH fraction (2.6 kD) and a series of four HMWH fractions (about 20 kD) with varying degrees of polydispersity, charge density and anticoagulant activity. bFGF, a prototypic heparin-binding angiogenic growth factor, was injected i.p. at 220 pM on days 0-4. The heparins were given s.c. on days 0-13 or 0-14 at doses which were approximately within the range used clinically. Angiogenesis was assessed by microscopic morphometry and image analysis in groups of animals killed on days 14 and 15. Compared with the saline control, the LMWH and three of the HMWHs significantly inhibited angiogenesis in terms of microvascular length (MVL), a measure of microvascular density. Interestingly, the vascularized area (VA), a measure of microvascular spatial extension, and the total microvascular length (VA x MVL) were significantly lower in the LMWH-treated animals than in the animals treated with one of the HMWHs. The total microvascular length was, moreover, significantly reduced in the LMWH-treated animals compared with the combined data of all the HMWH-treated animals. No significant effects were related to the degree of charge density and anticoagulant activity of the heparins. In view of the putative significant angiogenic role of bFGF in human angiogenesis diseases, the present findings may have implications for the choice of anticoagulant treatment modality for patients suffering from cancer and other angiogenesis diseases.