The objectives of this study are to uncover the mechanisms by which aerogenic infection with facultative intracellular parasites is overcome. Syngeneic rats will be infected with either L. monocytogenes or M. tuberculosis which produce acute and chronic lung disease, respectively. Rats will be infected either aerogenically, intravenously or intratesticularly in order to compare the fate of the parasite in various organs following different routes of inoculation. The progress of infection will be monitored by means of viable counts. The mechanisms underlying the responses to infection will then be analyzed in three parts. 1. The initiation of the immune response. The transport of bacilli from the portal of entry to lymphoid organs will be traced by means of viable counts. 2. The lymphoproliferative response will be masured by incorporation of 3H-thymidine into lymphocyte DNA. Adoptive immunity will be used to quantitate the output of specifically sensitized lymphocytes. 3. The effector mechanism. The assembling of lymphocytes and macrophages into the lungs will be studied by in vivo radioactive labelling techniques. By judicious timing of the pulse of 3HT it will be possible to separate the migration of lymphocytes and monocytes into the pulmonary DTH reactions elicited by inhalation of tuberculin as an aerosol. Cell migration will also be studied in rats adoptively immunized with lymphocytes radioactively labelled in vitro. Finally, the influence of local conditions in the lung, such as oxygen tension, will be examined.