A study has been made of the preservation of fine structure, phospholipids, and the activity of acid phosphatase and esterase in rat liver fixed in various solutions containing 4 per cent formaldehyde. Examination of methacrylate-embedded preparations shows that calcium-containing fixatives result in poor preservation of fine structure, whereas veronal-treated or phosphate-buffered formalin gives excellent results if the tonicity of the solutions is suitably adjusted by addition of sucrose. Formol-phosphate, to which Versene has been added, causes deterioration of cellular morphology. Phospholipids are retained almost quantitatively in tissue fixed in formol-calcium, and in phosphate-, collidine-, or triethanolamine-buffered formalin. About 50 per cent of the activity of acid phosphatase and esterase are preserved after 24 hours exposure to these fixatives at 0-2 degrees C, and the distributions of the enzymes and of phospholipids, as judged by cytochemical staining results, are not altered by any of these formalin solutions. Consideration of the morphological and biochemical integrity of the fixed tissue suggests that 4 per cent formaldehyde, buffered at pH 7.2 with 0.067 M phosphate, and containing 7.5 per cent sucrose, is the most suitable of the fixatives for combined cytochemical staining and electron microscopical studies.