It is well established that ISCOMs function efficiently as an antigen-presenting system and protective immunity has been evoked against a variety of infectious agents. The built-in saponin adjuvant from Quillaja saponaria Molina is responsible for the strong immunoenhancing activity displayed by the ISCOM. However, to allow the use of ISCOMs in human vaccines it is necessary to determine the immunological properties and toxicity of chemically defined Quillaja components. Thus, the present study was carried out in a mouse model to determine the adjuvant activity and toxicity of "free", isolated Quillaja components, as well as formulated into particles, i.e. ISCOM matrix. The purified Quillaja components and the ISCOM matrix formulations were examined for their adjuvant activity in a model system consisting of purified influenza virus antigen and Quillaja saponins. It was demonstrated that a Quillaja component, designated QH-C, either as a "free" component or in an ISCOM matrix, has a strong adjuvant activity, but little or no toxicity in the doses tested. In addition, QH-C in the form of ISCOM matrix does not induce any local reactions at the site of injection. Thus, ISCOMs containing the QH-C component, devoid of toxicity, but with strong adjuvant activity, can prove to be useful in adjuvant formulations for human use.