The Henderson apparatus (D. W. Henderson, J. Hyg. 50:53, 1952; 0. T. Miller and G. G. Gremillion, In Technical Manuscript 39, U.S. Army Biological Laboratories, Fort Detrick, Md., 1963) has become standard equipment for exposing a variety of animals to microbiological aerosols. When small animals such as mice are used, it is advantageous to aerosolize many animals in a single study. The standard small-animal exposure chamber supplied with the Henderson apparatus accommodates only 16 mice, necessitating repeated periods of nebulization. This produces an unavoidable variation in the character of the aerosol delivered to the different groups of mice. To circumvent this problem, we have devised a small-animal exposure chamber which holds 66 mice. It is designed for use with I.