Flavone acetic acid (FAA), an antitumour drug at present undergoing clinical trial for cancer treatment, has been found to activate natural killer cell activity in spleen cells of mice following in vivo treatment at doses of 45-330 mg/kg. The activity of FAA was measured using a 51Cr-release assay, employing the YAC-1 lymphoma line as a target cell population, and was found to be comparable to that of the interferon inducer polyinosinic acid-polycytidylic acid. The induction of activity was blocked by anti-asialo GM1 antibody. The time course of induction of activity by FAA was similar to that of the induction of haemorrhagic necrosis of the colon 38 tumour in mice, suggesting that natural killer cells or other components of the immune system may be involved in the action of FAA.