Abstract: Apparatus for aerosol immunization of subjects such as day old chicks and the like including an aerosol exposure chamber having controlled humidity, apparatus for providing a precisely controlled spray of vaccine in the exposure chamber at a predetermined droplet median diameter and feed rate, apparatus for continuously supplying subjects, such as chicks and the like, to and removing them from the chamber for aerosol treatment, and apparatus for maintaining the relative humidity in the exposure chamber in the vicinity of 100% to prevent significant evaporation of water droplets containing the vaccine which could cause such droplets to shrink to within the respiration range of the subjects, thus harming them.