Binding affinities of chemically modified heparins for human stem cell factor (SCF) were examined using fragmin/protamine microparticles (F/P MPs) and an enzyme-linked immunosorbent assay (ELISA). The binding of SCF to F/P MP-coated plates was inhibited with high concentrations of heparin and fragmin, but not others. The binding of SCF was also inhibited with 0.55 M or higher concentrations of NaCl in the medium. These results suggested that a high content of all three sulfate groups in repeating disaccharide units is required for interaction with SCF. Furthermore, pre-immobilized SCF on F/P MP-coated plates significantly stimulated proliferation of a human erythroleukemia cell line.