In recent years, a significant number of human cases of Q fever have occurred in the United States, some of which have been attributed to the consumption of milk. Upon investigation, it was ascertained that Q fever in cattle, goats, and sheep is endemic in certain western and southwestern States. It was observed that the danger of transmission of the disease to man could be reduced by slight changes in the procedures for pasteurizing milk. Because of the need for precise information as to the pasteurization times and temperatures necessary to assure destruction of the Q fever organism, the Public Health Service, in cooperation with the University of California, established a research project ot study the thermal destruction of this organism. Details about the research project and the new pasteurization recommendation which resulted from it are described in this monograph.