The specific objectives of this proposed research are: To more accurately define the limits of specificity of the cholesterol-digitonin reaction in tissues; to compare and contrast the artifacts observed in tissue fixed in glutaraldehyde-digitonin with in vitro mixtures of various phospholipids, cholesterol (and other 3-B-hydroxysterols) and proteins; to examine the fine structure of cholesteric gallstones in mice (Tepperman diet) and in humans; to examine the localization and possible association of cholesterol to hepatic organelles in mice (Tepperman diet); to analyze with H3 digitonin the relative distribution of cholesterol in a variety of organs (intestine, liver, gallbladder, adrenal, brown and white fat, blood vessels) in experimental (Tepperman diet) and control mice; and to analyze the distribution of cholesterol in developing, degenerating and regenerating myelin in rat sciatic nerve. Cholesterol localization will be determined by processing the tissues outlined above in a fixative containing H3 digitonin to form a sparingly soluble cholesterol digitonide. An analysis of tissue fine structure and correlated autoradiography will be the methodology employed as a basis for observations.