A new theoretical model for estimating yield of initial DNA strand break induced by several monoenergetic electrons is presented. It is based on the Monte Carlo track structure simulation and on new DNA structure models (one turn of double-strand DNA, nucleosome, solenoid), and links physical and chemical stages of radiation action. Direct and indirect effects are strictly distinguished. Some results of calculations indicated: (1) the number of single strand breaks per nucleus (6 microns in diameter) per Gy in pure water was about 10 times that in a cell environment (OH radical life time is assumed to be 8.7 ns). This is due to the difference in the time-dependent variation in the total number of the OH radical; and (2) the contribution of indirect effects to total damage decreased as the order of the DNA target model structure used in the stimulation increased (e.g. a one-turn model of double-strand DNA, approximately 98.4%; but the 30-nm solenoid model, approximately 86.1%). This was due to the protective effect of histone protein against OH radical attack. Double-strand breaks were scored if two single-strand breaks were located on the same base pair. The present study indicated that the information from morphological and biochemical examinations of the cell environment must be considered more carefully with computer simulation.