We have attempted to determine the relative biological effectiveness (RBE) of negative pions in the Bragg-peak region as compared to the plateau region and to gamma rays. We irradiated LAF{sub 1} mice, bearing 5-day-old lymphoma ascites tumors, in the peak and plateau regions of a 90-MeV pion beam for 40 hours in temperature-controlled holders. The animals were then sacrificed; lymphoma cells were withdrawn and titrated into adult female LAF{sub 1} mice. The proliferative capacity of the irradiated tumor cells was evaluated after 8 weeks by observing the percentage of animals developing ascites tumors. Surviving fractions were then calculated from LD{sub 50}`s of control and irradiated animals. Radiation doses in the 50 plateau region were measured with LiF dosimeters calibrated against cobalt-60 gamma rays. We calculated peak doses from those at the plateau, using a measured average peak-to-plateau ionization ratio of 1.5. Doses in the plateau region ranged from 145 to 250 rads; doses in the peak region ranged from 220 to 380 rads. The survival curve for cells irradiated in the peak region gave a D{sub 0} of 65 {plus_minus} 15 rads. The plateau points were not reliable. A replicate experiment was performed using Co{sup 60} {gamma}-rays, yielding a survival-curve …