RTS, S/AS01, a vaccine targeting pre-erythrocytic stages of Plasmodium falciparum, is undergoing clinical trials. We report an analysis of cellular immune response to component Ags of RTS, S—hepatitis B surface Ag (HBs) and P. falciparum circumsporozoite (CS) protein—among Tanzanian children in a phase IIb RTS, S/AS01 E trial. RTS, S/AS01 E vaccinees make stronger T cell IFN-γ, CD69, and CD25 responses to HBs peptides than do controls, indicating that RTS, S boosts pre-existing HBs responses. T cell CD69 and CD25 responses to CS and CS-specific secreted IL-2 were augmented by RTS, S vaccination. Importantly, more than 50% of peptide-induced IFN-γ+ lymphocytes were NK cells, and the magnitude of the NK cell CD69 response to HBs peptides correlated with secreted IL-2 concentration. CD69 and CD25 expression and IL-2 secretion may represent sensitive markers of RTS, S-induced, CS-specific T cells. The potential for T cell-derived IL-2 to augment NK cell activation in RTS, S-vaccinated individuals, and the relevance of this for protection, needs to be explored further.