The paper takes issue with the use by glutaraldehyde suppliers of the term 'for electron microscopy', and the common practice of researchers giving insufficient or no data about the glutaraldehyde they use. Investigation of 11 commercial glutaraldehydes recommended for electron microscopy shows that only three or four of them are adequate for this purpose, using criteria set forth in papers dated between 1965 and 1989. The present paper reports that a check of purity can best be done by spectrophotometry. The 234/280 or 235/280 nm absorbance ratio is a precise indicator of the degree of polymerization, provided certain conditions stated in this paper are fulfilled. Some of the storage precautions taken by, or proposed by, suppliers are superfluous, and only mask the inadequate purification by the suppliers. A simple protocol for the storage of stock solutions is given. Alkaline glutaraldeyhyde is inherently very unstable, even in the refrigerator. Fixatives should, therefore, be stored in the freezer or should be freshly prepared.