The microstructures of transparent gels from ovalbumin, hen egg white lysozyme, and bovine serum albumin were examined by high-resolution scanning electron microscopy. Three transparent gel matrices were clearly found to be composed of a common structural unit in which heat-denatured molecules formed fine networks of linear aggregates. The molecular conformation of heat-denatured proteins in linear aggregates produced after heat treatment in the absence of salt was studied by measuring circular dichroism spectra, intrinsic tryptophan fluorescence, and adsorption of the dye 1-anilinonaphthalene-8-sulfonate. These experiments showed that all three heat-denatured proteins did not take on a random-coiled state but rather remained partially folded, with some hydrophobic regions becoming exposed to the solvent environment. This conformation could thus be categorized as the ''molten globule'' state.