The birefringence of fresh and fixed mouse pancreatic acinar tissue was studied, utilizing whole mounts of pancreas from which the mesentery had been removed. Fresh pancreas in Tyrode's solution demonstrated positive birefringence with respect to the radial axis (the axis radiating from the nucleus as spokes from a wheel). Formol fixation reversed the sign of birefringence to negative with respect to the radius. The magnitude of birefringence increased with longer fixation. Neutral formol also reversed the sign to radially negative, but the magnitude did not increase with longer fixation. Tissue fixed in 2 per cent osmium tetroxide or potassium permanganate demonstrated strongly negative birefringence with respect to the radius. The cytoplasm of tissue fixed in acetic acid, though finely granular, still possessed radially positive birefringence. Birefringent areas could be seen in tissue fixed in potassium dichromate, but the sign of birefringence could not be determined. Ethyl alcohol, chromic acid, picric acid, and mercuric chloride all produced a brilliant cytoplasm in which no birefringence could be demonstrated. Freezing markedly decreased the radially positive birefringence of fresh tissue. Fresh tissue placed in increasing concentrations of glycerol demonstrated increasing radially positive birefringence. When formol-fixed tissue was placed in glycerol, the radially negative birefringence decreased. Osmium tetroxide-fixed tissue in 50 per cent glycerol was isotropic. The granularity of ethyl alcohol-fixed tissue disappeared in 50 per cent glycerol, and radially positive birefringence was evident. Frozen tissue showed increasing radially positive birefringence in increasing concentrations of glycerol. The results are discussed in relation to theories of fixation.