Using mammalian cells in culture, biological intercomparisons will be made of the fast neutron beams currently used for radiotherapy. The properties of neutrons will be compared with other high LET particles used or proposed for radiotherapy. The electron affinic drugs will be tested as radiosensitizers and cytotoxic agents. OBJECTIVES: a) The overall objective of the program is the measurement of radiobiological parameters of relevance to high LET radiotherapy. Particular emphasis is placed on a comparison of the properties of the various particles in use or proposed for radiotherapy, including neutrons, pions and heavy ions. The initial plan called for neutrons to be studied in the first year of the grant, with the emphasis turning to pions and heavy ions in later years. This plan has been modified for two reasons. First, the intercomparison of clinical neutron sources has taken much more time and effort than anticipated as additional sources came into use. Travel expenses for these experiments were defrayed by CROS. Second, the heavy ion studies at the Bevalac have been postponed because adequate travel funds were not available in the later years of this grant. Instead, the experiments with electron-affinic drugs, proposed in the original grant application have been greatly extended and amplified. b) All goals set for the current year have been met, except for the experiments at Bevalac. Intercomparisons of the relative biological effectiveness were performed at Cleveland, the Texas A&M Variable Energy Cyclotron, the Naval Research Laboratory and also at the Hammersmith Hospital in London. As planned the OER for neutrons was measured for 101 MeV protons on beryllium.