MAC 16 is one of a series of mouse colon tumours originally induced by dimethylhydrazine. It is a relatively slow growing subcutaneous adenocarcinoma which becomes necrotic as it grows and causes severe body wasting in the host. This study has indicated that the tumour is resistant to a large number of standard anti-cancer drugs but is highly responsive to the investigational agent flavone acetic acid (FAA). The levels of FAA achieved in tumours are lower than those necessary for activity in vitro suggesting its mechanism of action in vivo is not direct cytotoxicity. Responding tumours demonstrate massive tissue necrosis and those which are not cured have viable tumour cells associated with tumour blood vessels. The anti-tumour effects are accompanied by control of the host's cancer cachexia. The unique chemosensitivity of MAC 16 to FAA suggests that this agent has a novel mechanism which may be dependent upon specific biological characteristics of tumours.