Water-soluble and acid-soluble pectic polysaccharides with significant immunostimulating activity were isolated from alcohol-insoluble solids (AIS) of leek. Pectic polysaccharides characterized with high anhydrouronic acid content and common pectic sugars – L-arabinose, D-galactose, D-glucose, and L-rhamnose. It was accomplished that D-galactose content was the highest, which allowed the assumption that pectin sugar side chains were from galactan type. Enzyme modification of acid-extracted pectin was realized. Different pectin-degrading enzymes were used. The first modified pectic fraction, obtained after endopolygalacturonase action and alcohol precipitation contained rhamnogalacturonan I and had high D-galactose content. The second fraction obtained from the first, after β-galactosidase treatment characterized with low D-galactose content. Probably this was result from β-galactosidase hydrolysis of pectin side chains. It was proved by the action of β-galactosidase that galactan side chains were mainly composed of β-(1→4)-linked D-galactose. It was accomplished that rhamnogalacturonan I had higher immunostimulating activity than the initial pectic polysaccharide, which confirmed the statement that pectin hairy regions were responsible for the biological activity. The second fraction had lower D-galactose content and lower activity than the first. Probably galactan side chains were responsible for the immunostimulating activity.