Immunologically active polysaccharide fractions of M.Wt. (1 — 4)-104 daltons were isolated from neutral, alkaline or acidic water extracts of Caltha palustris L. They were characterized using chemical analyses, IR and 13C NMR spectra. Immunomodulatory effect was estimated in vitro in granulocyte activation, lymphocyte activation and lymphocyte mitogenic tests.