Immunostimulatory polysaccharides can be preferentially extracted from food-grade microalgae using an aqueous ethanol extraction procedure. The resulting preparations exhibit extremely potent immunostimulatory activity. The preferential extraction of these immunostimulatory polysaccharides is dependent on the concentration of ethanol used and the extraction temperature. The most efficient conditions are 50% ethanol concentration at temperatures between 60.degree. and 70.degree. C. The isolated polysaccharide preparations are potentially useful as a botanical or pharmaceutical preparation to improve immune function.