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Gamma-inulin (g-IN) is a polymorph identified as the active component of inulin preparations that specifically activates the alternative pathway of complement (APC). The APC is central to many leucocyte functions, including B cell activation. We show here that g-IN, when formulated as a pure, endotoxin-free, fine suspension insoluble at 37°C and given at 50-100 μg per mouse, is a potent adjuvant for both humoral and cell-mediated responses to a variety of antigens. g-IN increased secondary IgG responses five- to 28-fold (P<0.001), using as antigen phosphorylcholine coupled to keyhole limpet haemocyanin; subclasses IgG 2a, 2b, and 3 were boosted several hundred-fold, IgG 110-fold. IgM and IgA were increased four- to six-fold. Delayed hypersensitivity, by footpad swelling after secondary challenge with sheep red blood cells (SRBC), was increased more than two-fold (P<0.001) if g-IN was included with the primary SRBC, equivalent to increasing primary doses 10-fold. g-IN was equally active if given 5 days before the primary SRBC. Thus it is an immune stimulant rather than a depot or vehicle for antigen. Mice primed subcutaneously with 30-300 HA units of H2N2 influenza virus (strain A/JAP) and challenged intranasally with a lethal dose of H1N1 virus (strain A/WSN) all died, but if g-IN was given with the primary antigen 50% of the mice survived (P<0.001), a deduced but not proven boost to cytotoxic T cell-mediated immunity. Unpublished work has shown that g-IN has no adverse effects at adjuvant-active doses. g-IN is thus a promising new vaccine adjuvant. It also has a potential for antitumour therapy, and is a specific reagent for exploring the role of complement in vivo. [ABSTRACT FROM AUTHOR]