The level of serum lysozyme in mice varies in relation to the dose of irradiation in several phases. High doses result in a progressive decrease whereas a decrease on the first and about the ninth day is observed after irradiation with lower doses. A rise in lysozyme level occurs on the third day. Enhancement of lysozyme levels was observed after application of 20-50 r. Administration of lysozyme did not affect the deaths of mice receiving with massive doses of irradiation. © 1966 Institute of Microbiology, Academy of Sciences of the Czech Republic.