The water extract of the lettuce Lactuca sativa, but not the ethyl acetate extract, inhibited the growth of HL-60 leukaemia cells and MCF-7 breast cancer cells. This correlated with the activation of checkpoint kinase 2 (Chk2), the induction of the tumour suppressor p21, and the severe downregulation of the proto-oncogene cyclin D1. The ethyl acetate extract, but not the water extract, induced HL-60 cell death, which correlated with the acetylation of alpha-tubulin. The acetylation of alpha-tubulin is indicative for microtubuli stabilisation such as induced by taxol. The calculated amount for human intake would require approximately 3 kg lettuce to reach the required concentration shown to inhibit 50% HL-60 proliferation.