[en] The distribution of particle track lengths is related to particles which penetrate into the said volume from the surrounding medium and particles which originate inside the considered volume. Relations are given for probable track lengths. Particles which cause individual ionizing events are divided according to the beginning and end of their track in the cavity into internal, beginning, ending and passing. Relations are given for the frequency distribution of energy of these particles as are relations for the frequency and dose distributions of linear energy. On the basis of the said analysis a programme has been written for the calculation of microdosimetric parameters for a spherical cavity whose content consists of the biogenic elements H, C, N, and O. The calculated contributions of the said particles are presented in figures and related to the frequency distribution of linear energy. It was found that for a cavity of a diameter of 1 μm in soft tissue and irradiated with neutrons from a ²⁵²Cf source, passing particles comprise 92.8%. The share of ending and beginning particles is roughly eqUal. Under the same irradiation conditions with an enlargement of the cavity diameter to 3 μm, passing particles make up 81.5% and internal particles 3.6%. In case of irradiation with 0.1 MeV neutrons and a cavxty diameter of 1 μm, beginning particles make up the biggest proportion of 35.7% and passing particles take the smallest share of 12.4%. In the frequency distribution of internal particles, discontinuities have appeared corresponding to oxygen and carbon ions. (E.S.)