Aqueous extracts from whole dried mistletoe (Viscum album L., Iscucin) are often used in anti-cancer treatment. We studied the effect of extracts obtained from mistletoe bushes that grew on different host trees on bladder cancer cells by means of MTT-colorimetric cell proliferation/survival assays. The extracts possessed concentration-dependent cytotoxic properties whose extent varied with the host tree, but did not always correlate with the corresponding mistletoe lectin content. A 2-hours treatment of bladder cancer cells triggered a later, strong cytotoxic effect. This prolonged effect suggests that instillation with Iscucin has therapeutic potential for bladder cancer patients.