A complex, water soluble polysaccharide fraction having potent immunostimulatory activity isolated from Aloe vera. The polysaccharide fraction has an apparent molecular weight above 2 million daltons. Its major glycosyl components are glucose, galactose, mannose and arabinose. The invention further includes pharmaceutical compositions containing the instant polysaccharide fraction, optionally in combination with acceptable pharmaceutical carriers and/or excipients. These pharmaceutical compositions may be used to provide immunostimulation to an individual in need of such treatment by administering to such an individual an effective amount of the composition. The polysaccharide fraction is also useful as a component of dietary supplements and as a standardization component of commercial Aloe products.