There is considerable disagreement among the various investigators of particle retention in the lungs. The two major causes of lack of agreement are failure to use a uniform breathing rate and the use of a mean particle size to describe a test aerosol which has a wide-size distribution. Both the experimental and theoretic results show a minimum of retention in the 0.4 to 0.6 [mu] range. All of the data also indicate 90 per cent or more retention of particles greater than 7 [mu]. In addition to particle size, the next largest variable affecting retention is breathing rate. The theoretic calculations of Landahl agree well with the experimental results obtained by Landahl and co-workers, except for particles in the 2[mu] range. For this size, the theoretic calculations give results that are too high at both breathing rates. The use of radioactive sodium chloride to obtain an approximation of alveolar retention gives an excellent measurement for therapeutic application of aerosols in the smaller branches, bronchioles, and alveoli. To establish definitely the particle size for maximal retention, the experiments should be repeated, using a nonhygroscopic carrier. The results of the glycerol-water aerosols indicate that the maximum occurs for particles which have an equivalent diameter of 1.5 [mu]. By assuming that the particles grow until they contain 85% water, the diameter for the maximum would increase only to 2.3 [mu].