Menadione (vitamin K3, 2-methyl-1,4-naphthoquinone) is a synthetic derivative of napthoquinone. Its ability to inhibit cell growth in a wide variety of and human tumor cell types, and in rat hepatocytes has been recognized. Using a rat transplantable hepatoma model, we have evaluated the cytotoxic activity of menadione in hepatoma cells. Tumor cells in culture were sensitive to menadione treatment. The ID50 of drug is 3.4 microM as shown by a colorimetric MTT (3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. Tumor-bearing rats were randomized into the treatment (n = 16) and control (n = 15) groups. Rats in the treatment group received intraperitoneal injection of menadione (10 mg/2 ml) once a week for four times; the control group received 2 ml water instead. None of the control rats survived after the 17th day following the start of treatment, while 5 out of the 16 treated rats responded well and survived long-term (greater than 60 days). Medadione was shown to inhibit actively the growth of hepatoma cells in vitro as well as in vivo.