SJOSTRAND has shown that renal proximal tubule cells of several mammalian species contain autofluorescent granules with a characteristic segmental distribution within the proximal tubule1. Electron microscopic observations of proximal tubule cells have demonstrated several structurally different varieties of cytoplasmic granules and vacuoles2–6. Although none of these has been directly identified with the autofluorescent granules, I have observed that one type of dense granules has a very similar distribution along the proximal tubule7. Novikoff and Koenig have pointed out that acid phosphatase positive granules seen by light microscopy seem to correspond to the autofluorescent granules8,9. Direct information has now been obtained concerning the identity and ultrastructural properties of the autofluorescent granules by isolation of a fraction from rat kidneys in which they are highly concentrated.