[en] The determine the relative biological effectiveness (RBE) of mixed neutron-gamma fields, nuclear reactor radiations have commonly been used. In such studies, however, there is a wide variation in gamma - to - neutron ratios and, hence, the mixed radiation is not readily characterized. On the other hand, RBE values for mixed neutron-gamma radiations have been estimated using total doses in the range of 200 to 600 rad. No information is available for lower total doses (in the range of 5 - 20 rad). This paper concerns the assessment of RBE of mixed neutron-gamma radiation at various proportions of its constituents (neutron components of 10, 25, 50, or 75%) on mice given total doses of 5, 10, or 20 rad, in terms of conformation-shift effects in blood-plasma protein fractions, as determined from measurements of blood-plasma electroconductivity and dielectric constant. RBE values were found to vary from 1.25 to 3.70 at neutron-component proportions of 10 to 75% in mice exposed to 4.1 MeV neutrons (plutonium-beryllium source) and gamma rays (radium source). (author)