Microfilaments (MF) are present in a variety of animal and plant cells, and there is substantial evidence that they are involved in cellular motility and contractility (1). Although MF's were first discovered by electron microscopy (EM), they are not distinctly visible in routine EM preparations of whole tissue. In partially disrupted hepatocytes and in isolated bile canaliculi, however, the MF's are prominently contrasted with the uranyl acetate en bloc treatment (2). We now report our observations on the treatment of glutaraldehyde- (GA) fixed rat liver with various detergents. This procedure, followed by uranyl staining, increases markedly the electron density and the contrast of MFB's in tissue sections.Materials and Methods: Livers of adult rats were fixed by perfusion with 2.5% purified GA in 0.1M Na-cacodylate buffer, pH 7.2, and 0.05% CaCl2 for 10 min. 50-μm chopper sections were treated with 0.1-1% Triton X-100 or sodium deoxycholate (DOC) for 1 h at 25°C, rinsed briefly with the buffer, and then postfixed in 2% aqueous OsO4 for 60 min. at 4°C. This was followed by en bloc staining for 60 min. with 1% uranyl acetate in 0.2M Tris-maleate buffer, pH 5.2, and embedding in Epon. Ultrathin sections were stained with uranyl acetate and lead citrate. In control preparations the detergent treatment was omitted, but all other procedures were identical.