Inspections of microfilms have recently revealed blemishes which apparently developed 2 to 20 years after the films were processed. Most of the blemishes are small spots, usually reddish or yellowish in color, ranging from about 15 to 150 microns across. These blemishes have been classified on the basis of size, shape, color and character. This publication describes and gives colored illustrations of the various types, describes the method of observing them and recommends sampling procedures for microfilm inspectors. The cause, the exact mechanism of formation of the various types and generally accepted preventive measures are not as yet known. This publication is intended to promote uniform terminology, inspection and reporting.