RTS,S/AS01, a vaccine targeting pre-erythrocytic stages of Plasmodium falciparum, isundergoing clinical trials. We report an analysis of cellular immune responseto 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/AS01E trial. RTS,S/AS01 E vaccineesmake stronger T cell IFN-γ, CD69, and CD25 responses to HBs peptides than docontrols, indicating that RTS,S boosts pre-existing HBs responses. T cell CD69and CD25 responses to CS and CS-specific secreted IL-2 were augmented by RTS,Svaccination. Importantly, more than 50% of peptide-induced IFN-γ+ lymphocytes were NK cells, and the magnitude of the NK cellCD69 response to HBs peptides correlated with secreted IL-2 concentration. CD69and CD25 expression and IL-2 secretion may represent sensitive markers ofRTS,S-induced, CS-specific T cells. The potential for T cell-derived IL-2 toaugment NK cell activation in RTS,S-vaccinated individuals, and the relevanceof this for protection, needs to be explored further.