The ability of field bean protease inhibitor (FBPI) to inhibit ethylnitrosourea (ENU)-induced tumours of the nervous system of Sprague-Dawley rats was investigated. Groups of 1-day-old rats were injected intraperitoneally (i.p.) with neurocarcinogenic amounts of ENU and a few hours later, one group was treated i.p. with 80 mg of FBPI per kg body weight. This treatment was carried out three times a week for the first month and five times a week for the next month. Animals were killed when they were neurologically ill and their neural tissues were assessed for lesions. Those FBPI-treated rats which showed no illness were also killed to terminate the experiment about 8 weeks after the last rat of the control group was affected with paralysis. The neural tumours induced in all groups were predominantly large tumours found in the cerebrum of the rats. ENU-treated rats showed a 100% incidence of nervous system tumours with a mean time of manifestation of neurological symptoms of 282 days, which was significantly shorter in comparison to that noted in the FBPI-treated group. The latter group showed an incidence of 58.3%, i.e. a significant reduction of 41% in the incidence of neural tumours, as well as a lower mean value for the number of tumours per rat. All these aspects indicated that FBPI is a potential neurooncopreventive agent. A neural tumour incidence of 100% in the rats treated with heat-inactivated FBPI confirmed that the tumour suppressive activity of FBPI is related to its protease inhibitory activity.