The electron spin resonance technique has been used to study the production of long-lived radicals in glycine, alanine, valine, aspartic acid and glutamic acid. The yield of radicals as a function of dose was measured, using as a source of radiation 14·3 MeV electrons at a very high dose rate. In all cases saturation was observed at high doses (in the megarad range). The yield vs. dose curves were analysed in terms of exponential functions and some possible explanations put forward. Absolute values of the yield per unit dose, as well as the saturation values, were calculated, and possible sources of error in these values were discussed.