Work was completed on the high-temperature, high efficiency air filter. Technical assistance was given at a large-scale, privately financed run of an all-glass medium. Full-size filers have been produced in quantity and are offered by manufacturers; it is considered that commercial manufacture has been established. A survey was made of air sampling practices at 37 laboratories. Analysis of the survey results indicated that 22 different kinds of air-sampling filer media were in use among the laboratories questioned. We have recommended that air-assay practice be simplified by use of fewer media. A group of five media has been proposed as adequate for meeting all requirements. Laboratory work was done on the development of a high-efficiency, low-ash, all-purpose, air-assay paper. Two methods of approach were tried. In one an effort was made to produce cellulose fibrils in sufficient quantity and quality to act as the fine-fiber component of a filter. Only moderate success was attained. Better promise was shown by combining synthetic organic microfibers with cellulose fibers in a wet-formed sheet. No plant work was undertaken on this item.