Pathogen‐derived pattern recognition ligands like lipopolysaccharide (LPS) and bacterial cytidine–guanosine (CpG)‐DNA not only activate dendritic cells and macrophages but are also mitogenic for B cells. Less clear are the claimed effects of CpG‐DNA on T cells, which range from direct activation, costimulation, or indirect transient activation via antigen‐presenting cell (APC)‐derived interferon type I (IFN type I). Here we demonstrate that CpG‐DNA sequence specifically triggers macrophages to produce IFN type I, interleukin (IL)‐12, IL‐6 and tumour necrosis factor (TNF), but lacks the ability to directly costimulate T cells. Strikingly, poly‐guanosine (poly‐G) extensions to CpG‐containing oligonucleotides (ODN) abolished the macrophage stimulatory potential yet generated T‐cell costimulatory activities. In fact, independently of CpG‐motifs, poly‐G‐ODN displayed the ability to costimulate T cells. Costimulation was operative on CD8 T cells but not CD4 T cells. Poly‐G‐mediated costimulation resulted in IL‐2‐driven T‐cell proliferation and induced cytolytic T cells. Overall the data imply that poly‐G motifs costimulate antigen reactive CD8 T cells, while CpG‐DNA motifs fail to do so but may affect T‐cell activation via APC derived cytokines such as IFN type I.