Abstract: A process for preparing gamma inulin comprising the steps of (a) recrystallizing crude inulin from water at a temperature below 37.degree. C. to obtain a suspension, (b) heating the suspension at a temperature of from about 25.degree. to 45.degree. C. for about 1-3 days, (c) further heating the suspension at a temperature of about 40.degree. to 55.degree. C. for about 0.5 to 1.5 hours, and (d) isolating insoluble gamma inulin from the suspension. A composition comprising particles of inulin or an inulin derivative in the gamma polymorphic form is characterized in that the particles have a low rate of solution in aqueous media above 30.degree. C., particularly above 37.degree. C. The composition is effective as the active component of an immunotherapeutic preparation for activation of the alternative pathway of complement, or for antitumor treatment.