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Meta l impregnat ion or ta in , of tissue sections for electron microscopy has become the accepted practice in recent years. Salts of metals of h igh atomic weight such as u ran ium, chromium, thor ium, lead, or tungsten, among many tested, have been found suitable (6, 1). Lead , as prepar ted by Watson (6), is now widely used, hu t this solution is extremely unstable in air and becomes covered by a meta l l ic-appear film which imparts to the s ta ined section a well known deposit of e lectron-opaque particles and crystals, reducing considerably the percentage of clean areas suitable for micrography (Fig. l) Several procedures and devices have been described to lessen these contaminat ions. Peachey (5), using lead hydroxide, or subacetate, found tha t if the solution is kept in a syringe with the protect cap filled with sodium to absorb the CO2 of the atmosphere, con tamina t ion is reduced. More recently, a fairly unwieldy appara tus has been suggested for the same purpose (4). Tests performed in our laboratory in which s t a in ing was a t tempted in a chamber under cont inuous ni t rogen flow and in the presence of a ba r ium chloride t rap were not completely successful, which is to say tha t con tamina t ion cont inued to be a problem. A n u m b e r of o ther var iat ions have been recommended. Lever (2), e.g., described a method for prepar a s t a in ing solution by adding potassium to a lead solution. To dissolve some of the occasionally formed crystals of contamina t ion the sections are rinsed afterwards for a few seconds in a weak potassium solution; this step is very critical because the ta in ing is also weakened by the alkali and uni form staining is difficult to achieve. These various shortcomings of current ly avai lable methods have st imulated us to search for a lead salt tha t would not be affected by the components of the atmosphere. Among several heavy metal salts tested, the above-ment ioned lead hydroxide of Watson seemed to be the most effective in ta in . Exper iments with the commercial ly available product showed that this salt would give no impregna t ion when dissolved in water, or in Institute. Dr. Millonig's present address is Biological