IN A number of historic developments within the past two decades, the microorganism known as the Eaton agent was shown to be a principal cause of primary atypical pneumonia in man.<sup>1-14</sup>Originally considered to be a virus,<sup>5</sup>it was later characterized as a filterable agent which grows extracellularly with the properties of a pleuropenumonia-like organism (PPLO).<sup>15-19</sup>It has been classified as<i>Mycoplasma pneumoniae</i>.<sup>20</sup>Clinical, seroepidemiologic, and controlled therapeutic studies in military populations,<sup>21-27</sup>prison volunteers,<sup>28,29</sup>industrial employees,<sup>13</sup>pediatric ward patients,<sup>30,31</sup>and in college students<sup>32</sup>have indicated the important and pathogenic role of<i>M pneumoniae</i>in the causation of human respiratory disease and its susceptibility to tetracycline. Our study was undertaken to further determine among civilian adults the occurrence and significance of PPLO in patients with various types of respiratory illnesses. <h3>Materials and Methods</h3> From August 1963 to April 1964, 197 patients hospitalized for pneumonia in the medical wards of the