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Background Abelmoschus manihot (Linn.) Medicus is a traditional herbal medicine whose flowers, stems and leaves exhibit widely pharmacological activities. However, only the flowers have long been used as medicine while the stems and leaves were mainly discarded and burned, which undoubtedly caused enormous waste of these resources and serious environment pollution. Many researches have indicated that bioactivities of polysaccharides were significantly improved after sulfation. The aim of this study was to investigate the characterization and immunomodulatory activity of polysaccharides from stems and leaves of A. manihot and a sulfated derivative. Results A mixed neutral polysaccharide (SLAMP-a) and two acidic polysaccharides (SLAMP-c and SLAMP-d) were obtained from stems and leaves of A. manihot by DEAE-cellulose chromatography. SLAMP-a was a water-insoluble mixture while its sulfated derivative (S-SLAMP-a3), prepared with aminosulfonic acid, was a homogeneous polysaccharide with excellent solubility. The average molecular weights of S-SLAMP-a3, SLAMP-c and SLAMP-d were 1044.2 kDa, 477.8 kDa and 264.2 kDa respectively. SLAMP-a and its sulfate mainly contained glucose, and SLAMP-c and SLAMP-d were both composed of mannose, rhamnose, glucuronic acid, glucose, galactose, and arabinose. In vitro study indicated that S-SLAMP-a3, SLAMP-c and SLAMP-d exhibited significant immunomodulatory activity, while SLAMP-a showed little effects. Conclusion SLAMP-c and SLAMP-d from A. manihot stems and leaves could be explored as immunomodulatory agents, which would provide a way to utilize these enormously discarded resources and avoid massive waste. Additionally, the neutral polysaccharide, SLAMP-a, could also be developed after sulfation, suggesting that these disused resources would be further used effectively. [ABSTRACT FROM AUTHOR]