The reaction of colloidal ferric oxide with sialomucins (Hale stain) was applied to sialic acid-containing gangliosides in subcellular fractions of the brain, in order to investigate their localization by electron microscopy. Prior to all experiments, a check was made, by means of tritium-labeled ganglioside, to confirm that the ganglioside content in the subcellular particles was not the result of an unspecific adsorption during the isolating procedure. No considerable unspecific adsorption could be registered. Hale stain was first applied to subcellular fractions obtained from guinea-pig brains. Since nerve-endings, mitochondria and synaptic vesicles gave a positive staining reaction, incubations with neuraminidase and hyaluronidase were carried out in order to achieve a differentiation. Finally, the method was applied to the membranous cytoplasmic bodies (MCB) which were isolated from the brain of a patient with Tay-Sachs disease. It was concluded that: a) Hale stain stains gangliosides also, b) gangliosides are localized in the membrane of membranous cytoplasmic bodies and also very probably in the membrane of nerve-endings, c) synaptic vesicles give a Hale-positive reaction which cannot with certainty be attributed to gangliosides, d) the outer membrane of mitochondria contains Hale-positive substances, the nature of which is not known.