Peculiar electron dense granules were observed, in otherwise well preserved structures of the heart muscle and kidney of the rat after a 5-10 min perfusion, a 2 h fixation in the modified Karnovsky solution, followed by a postfixation in Millonig buffer containing 1% OsO4. The granules appeared most frequently on membranes, the surface of the red blood cells and in pinocytotic vacuoles, in stained and unstained sections alike. No artefacts were produced when the same buffer was used for both pre- and postfixation, or when the postfixative contained Veronal buffer.