Low temperature ultramicroincineration was employed to determine the morphological localization of "structure-bound" mineral and/or metallic elements within biological cells at the electron microscope level. This technique chemically removes organic material from thin sections of tissues with reactive, excited oxygen instead of heat as used in a furnace. The remaining ash representing the mineral/metallic ultrastructure of cells is advantageous for ultrastructural studies because incineration without applying heat is less destructive than the burning associated with high temperatures. This low temperature incineration method was applied to thin-sectioned avian shell gland mucosa, a calcium transporting system, as a sample tissue. The results include: recognition of many subcellular organelles in the ash patterns, identification of dense, ash-containing granules (possibly organic-metallic complexes) in epithelial cells which may be involved in calcium transport, description of ashed erythrocytes and collagen, comparison of ashed glutaraldehyde fixed tissue with and without osmium postfixation, description of lead-stained cells after ashing, demonstration that ash preservation is dependent upon section thickness, illustration of the fine resolution obtainable because the ash residues remain relatively near their in situ origins, discussion of technical problems in this relatively new field, and demonstration of the presence of Ca and P in the ash with electron microprobe X-ray analysis.