Reduced lytic function of cytotoxic cells in CFS/ME. In vivo assessment of NK and CD8+T cell lysis (cytotoxic activity) of tumour cell lines K562 and P815 respectively in CFS/ME (black bars) in comparison to controls (white bars). Lytic activity is represented as percent (%) lysis on the y-axis. *Denotes statistical significant results. Data presented as mean ± SEM.