Algammulin is a suspension of 1-2 microns ovoids of the immune stimulant gamma inulin (g-IN) in which alum is embedded as a carrier for protein or other anionic antigens. Tests for specific IgG and seroconversion responses in mice immunized with keyhole limpet haemocyanin (KLH) show that the presence of g-IN on the alum has increased its adjuvanticity 6- to 17-fold (p less than 0.001), and thus has a synergistic effect. A mixture of alum and g-IN was no more active than alum alone. Low KLH doses at borderline seroconversion levels (0.1-1 microgram/mouse) allow comparison with a vaccine situation. The improved Algammulin responses extended to 58 days after primary doses and to memory recall after boost at 65 days. At 1 mg Algammulin i.p. in primary and secondary doses the anti-KLH IgG responses were equal to those from Freund's complete adjuvant. The g-IN on the alum increased all responses tested (IgG 1, 2a, 2b, 3 and total IgG, IgM, IgA and IgE), but changed the emphasis from that of alum (favouring mostly IgG 2a, IgG 2b and IgA).