This study was aimed at investigating the radiation induced DNA fragmentation pattern as a function of cellular differentiation and radiation quality. DNA double strand breaks (DSB) induced by gamma-rays were analyzed in K562 human proerythroblasts before (AP cells) and after (D cells) differentiation induction while DNA DSB induced by 125 keV/micrometers N-ions have been studied in AP cells. Pulsed-Field Gel Electrophoresis (PFGE) of cellular DNA was used to determine the DSB yield by analysis of the Fraction of Activity Released (FAR) and of the fragmentation pattern in a specific size range (5.7-0.225 Mbp). The results so far obtained show that the DSB induction by gamma-rays is different if evaluated with the FAR or with the fragmentation analysis. The DSB yield obtained with the former method is about 1.4 times higher in AP respect to D cells while the latter method indicates that more fragments are produced in D cells. Comparison between gamma-rays and N-ions in AP cells shows that no significant differences are detected by the FAR analysis; otherwise fragmentation analysis demonstrates a higher effectiveness of nitrogen ions.