Grinding samples of Baltic amber from 2-3 mm grains to 20-30mu particle size increased the intensity of electron spin resonance signal by 6.3. This clearly indicates the existence of free radicals in the ground sample. The phenomenon of formation of free radicals by mechanical action is discussed also for other cases (charcoal, colophony, adhesion of polished metal surfaces, cellulose, polyvinyl chloride, polystyrene, nitrocellulose, etc.). -- AATA