The use of gallic acid derivatives in the visualization of osmium in tissue sections has been re-investigated. By the use of alkyl esters of gallic acid greatly improved results can be obtained. Fixation with buffered osmium textroxide followed by ethyl gallate affords a simple and reliable method for staining fat droplets, mitochondria, etc. According to the hypothesis put forward the distribution of osmium is determined chiefly by the distribution of unsaturated fatty acids; none is bound by nucleic acids and relatively little by protein. The result is claimed to be an histology based primarily on lipids, which is contrasted with the customary histology based on nucleic acids and proteins. Evidence is given that osmium tetroxide causes polymerization of unsaturated lipids by the cross-linking of ethylenic double bonds. This is particularly liable to occur in layers of oriented lipids. Such layers are widely distributed in living cells; their stabilization by linkage through osmium is considered to play the most important part in cytological fixation by osmium tetroxide.