Structural requirements of heparin-like molecules for binding of MSP1-42. Binding of MSP1-42 to heparin-agarose beads was inhibited by heparin but not by de-N-sulfated (de-NS-) and de-6O-sulfated (de-6OS-) heparin. Binding of MSP1-42 was also inhibited by K5 polysaccharides with high-level N- and O-sulfation (K5-NSOS-H), but not by other K5 polysaccharides having different sulfation levels or patterns, or by EK5 polysaccharides. Heparin-agarose bound proteins were detected by Western blot using antibodies to MSP1-19. K5-NSOS-H was the most inhibitory molecule in parasite invasion inhibition assays, whereas the other molecules had little or no invasion-inhibitory activity.