The induction of protective immunity with recombinant vaccines is dependent on the identification of adjuvant or delivery systems that can augment immune responses, especially cellular immune responses, to soluble protein antigen. In this study we demonstrate that an adjuvant formulation comprising QS21, a purified form of saponin and 3D-monophosphoryl lipid A (MPL), a nontoxic derivative of lipopolysaccharide (LPS), enhances cellular and humoral immune responses to a recombinant HIV protein. Analysis of cytokine secretion by antigen-specific T-cells from the spleen demonstrated that QS21 augmented Th1 and Th2 responses, whereas addition of 3D-MPL resulted in preferential induction of type 1 T-cells. Furthermore, analysis of the subclass of the IgG antibody in the serum in mice immunized with gp120 with the combined adjuvant formulation confirmed the selective activation of Th1 cells in vivo. 3D-MPL was found to enhance B7-1 expression and IL-12 production by macrophages, which are known to be involved in the LPS-induced enhancement of Th1 responses. Thus 3D-MPL appears to act as an adjuvant, without the toxicity associated with LPS, by controlled and selective potentiating effects on antigen presentation and T-cell activation.