To assess the usefulness of oolong tea extract (OTE) staining for connective tissue observation, we examined the visceral pleural mesothelium of rat lung by transmission electron microscope. Four kinds of electron microscopic staining methods (routine, tannic acid, OTE in distilled water, and OTE in 0.1M phosphate buffer) were compared to determine the most suitable method for electron microscopic observation of ultrathin sections. Elastin (elastic laminae) was selectively stained by tannic acid and both water and buffered OTE. Except for elastin, connective tissue and cell ultrastructures were also electron enhanced by tannic acid and both water and buffered OTE staining. However, using water OTE, the electron-dense filaments (10–12nm in diameter) were obscured. In tannic acid staining, the unit membranes of the visceral pleural mesothelial cells were weaker as compared with routine and buffered OTE stains. Thus, the buffered OTE staining method is a highly useful technique for connective tissue observation and electron-enhanced staining in transmission electron microscopic preparations.