L929 cells were irradiated by cyclotron-produced neutrons and by 14.8 MeV monoenergetic neutrons. For comparison cells were also irradiated by 60Co gamma rays. Following irradiation cells were treated by an equimolar solution of deoxyribonucleosides, and the effect on cell survival measured. Results show that nucleoside treatment was efficient after low-LET irradiation: gamma ray survival curves were altered by deoxyribonucleosides in terms of significantly increased extrapolation numbers only, but without Do change. Cells irradiated by neutrons from either of the two sources did not respond to nucleoside treatment, and consequently their survival curves remained unaltered. These results show that the nucleoside effect does occur after low-LET irradiation, but apparently not following high-LET irradiation. Since deoxyribonucleosides as well as other cell breakdown products are released in irradiated and necrotic tumours due to massive cell destruction, such a nucleoside effect could possibly enhance the cell survival and thus effect the result of radiotherapy. Absence of the nucleoside effect in case of high-LET irradiation may therefore be an additional potential gain from neutrons in radiotherapy.