Boron neutron capture therapy has been carried out on BALB/c mice carrying the Harding-Passey melanoma s.c. on the thigh. p-Boronophenylalanine (BPA), a boronated analogue of natural melanin precursors, was used to target boron selectively to melanoma. BPA was administered to the mice either via i.p. injection or p.o. by intubation. 10B concentrations in tumor ranged from 15 to 40 ppm depending on the route and timing of administration. Irradiations with a predominantly thermal neutron beam were performed at the Brookhaven Medical Research Reactor. In the absence of BPA, only transient tumor growth delays were observed at low neutron fluences. At 5 x 10(16) n/m2, 4 of 22 tumors irradiated in the absence of BPA underwent long-term tumor growth control; after p.o. administration of BPA (40 ppm 10B in the tumor), the fraction of tumors controlled increased to 11 of 19. The average dose to the tumor in the latter group was 17.8 Gy, of which 14.8 Gy were due to the 10B neutron capture reaction. The biological effectiveness of the absorbed dose from the neutron capture reaction, at the 50% tumor control level, was found to be twice that of 100 kVp X-rays.