Developments have been made in the use of spraying methods in the preparation of specimens for examination in the electron microscope. At the same time, two wholly volatile diluents, containing electrolytes and adjusted to normalpH, have been developed for use in forming the spray drops. The use of volatile electrolytes makes it unnecessary to wash the specimen after the droplet patterns have been formed on the specimen screens.There are several qualitative and quantitative uses and advantages of the spray technique as described. The qualitative ones are these: (1) the brief drying time of the droplets helps to preserve the shapes of particles upon drying, and also allows studies to be made of rapidly reacting systems; (2) since the drop patterns are reproducibly representative samples of the suspension under investigation, qualitative assays of the particulate composition of the suspension can be made, and a comparison of the particulate composition of two closely similar suspensions can be made under ideal control conditions; (3) the droplet patterns are discretely bounded by blank areas of substrate film, and this fact makes practicable the detection of small and subtle differences between the fine structure of the specimen material and that of the substrate.The quantitative uses of the spray technique are: (1) by use of reference particles of known numbers in a suspension it is readily possible to determine the volumes of droplets issuing from a spray‐making device; (2) by use of reference particles it is also possible to make an assay of the number of other particles, such as a virus, per unit volume of mixed suspension; (3) if a suspension is highly purified, the particle weight of the material in suspension can be determined. Experiments involving the counting of about 10,000 particles show that the quantitative precision is as good as would be expected from the statistics of random sampling.The construction and use of a spray gun are described, as well as the characteristics of two volatile diluents: ammonium acetate and ammonium carbonate.