The discovery of aging blemishes on mocrofilm in storage has prompted a study of the statistical relationship of incidence to conditions of preparation and storage, the chemical and physical nature of the blemishes, and measures to be taken to prevent their formation. This paper is a report of progress in that study with recommendations based on current information. With few exceptions, reports of aging blemishes refer to negative silver-gelatin type microfilm in roll form. Extremely small amounts of information loss have been been reported. In some large collections of microfilm, no blemishes have been found. No evidence of biological attack has been found. Apparently, image silver is ionized, migrates, and is reduced to colloidal silver. The reaction often proceeds in the manner described by Liesegang for other silver-gelatin systems. In laboratory tests, blemishes formed in the presence of minute amounts of peroxides, certain imbibed chemicals and solid contaminats. Recommendations include: avoiding excessive densities on film, careful avoidance of physical damage to the image layer, the use of a small concentration of iodide ion in the fixer, thorough washing, uniform drying, storage at low temperature and humidity in sealed cans, careful avoidance of air-borne reactants, increased use of positive copies for archival storage, and careful periodic inspection of record films. -- CAL