To further increase the efficacy of malaria vaccine RTS,S/AS02A, we tested the RTS,S antigen formulated using the AS01B Adjuvant System (GlaxoSmithKline Biologicals). In a double-blind, randomized trial, 102 healthy volunteers were evenly allocated to receive RTS,S/AS01B or RTS,S/AS02A vaccine at months 0, 1, and 2 of the study, followed by malaria challenge. Protected vaccine recipients were rechallenged 5 months later. RTS,S/AS01B and RTS,S/AS02A were well tolerated and were safe. The efficacy of RTS,S/AS01B and RTS,S/AS02A was 50% (95% confidence interval [CI], 32.9%-67.1%) and 32% (95% CI, 17.6%-47.6%), respectively. At the time of initial challenge, the RTS,S/AS01B group had greater circumsporozoite protein (CSP)-specific immune responses, including higher immunoglobulin (Ig) G titers, higher numbers of CSP-specific CD4(+) T cells expressing 2 activation markers (interleukin-2, interferon [IFN]-gamma, tumor necrosis factor-alpha, or CD40L), and more ex vivo IFN-gamma enzyme-linked immunospots (ELISPOTs) than did the RTS,S/AS02A group. Protected vaccine recipients had a higher CSP-specific IgG titer (geometric mean titer, 188 vs 73 mug/mL; P < .001), higher numbers of CSP-specific CD4(+) T cells per 10(6) CD4(+) T cells (median, 963 vs 308 CSP-specific CD4(+) T cells/10(6) CD4(+) T cells; P < .001), and higher numbers of ex vivo IFN-gamma ELISPOTs (mean, 212 vs 96 spots/million cells; P < .001). At rechallenge, 4 of 9 vaccine recipients in each group were still completely protected. The RTS,S/AS01B malaria vaccine warrants comparative field trials with RTS,S/AS02A to determine the best formulation for the protection of children and infants. The association between complete protection and immune responses is a potential tool for further optimization of protection. Trial registration. ClinicalTrials.gov identifier NCT00075049.