We have investigated the effects of interferon-alpha (IFN-alpha) and 5-fluorouracil (5-FU) on meningioma cells in two different culture systems, evaluated by the uptake of radiolabelled methionine. With both IFN-alpha and 5-FU an inhibitory effect on the uptake of radiolabelled methionine by the meningioma cells was demonstrated, and we found a synergistic inhibitory effect with a combination of IFN-alpha and 5-FU. To obtain a maximal inhibition of cell metabolism without causing cell toxicity, we were able to decrease the dose of 5-FU by simultaneously adding IFN-alpha. Our results suggest that a combined treatment of IFN-alpha and 5-FU may be a successful alternative for patients with inoperable meningiomas. A novel in vitro positron emission tomography technique was used for the study of metabolic changes in tumour cells caused by drug treatment, which is complementary to conventional cell culture techniques.