Human prostate tumors are known to be good candidates for neutron therapy. The Dunning rat prostate tumor system R3327 was found in many studies to be an excellent model for human prostate tumors. There is still a paucity of studies on the response of the Dunning tumors to fast neutrons. Tumors of the R3327-HI subline are moderately well differentiated and mucin producing. They show one euploid cell population, a bromodeoxyuridine labelling index of 5%, a potential doubling time of 8.9 days, a volume doubling time of about ten days and a cell loss rate of 10%. Tumors were transplanted s.c. in the distal thigh of Copenhagen rats and treated with 60Co-photons (10, 20, 30, 40 Gy, 45 cGy/min) and 14-MeV-neutrons (8, 10, 12 Gy, 7 to 11 cGy/min). Tumor volumes were measured twice weekly. Growth delay was defined as time in days until the tumors reached twice their treatment volume. Linear regressions on the median growth delays of the different treatment groups were calculated. The ratio of the neutron- and photon-slopes yielded an RBE of 3.1 +/- 0.3. Additionally isoeffect-RBE values between 2.3 and 2.6 were graphically estimated.