EPR complex spectrum of 60Co irradiated crayfish cuticle persistent after 16 months. Mn2+ EPR spectrum of the crayfish cuticle represents an excellent internal reference. No significant influence of the KOH treatment on the irradiated cuticle EPR spectra. Irradiated crayfish cuticle EPR spectra show complex isothermal annealing behaviour. EPR spectrum of irradiated cuticle could represent an appropriate marker.