The ultrastructure of the organic matrix of demineralized urinary stones was examined by standard transmission and scanning electron microscopy as well as after malachite green-glutaraldehyde fixation. Crystal ghosts of both calcium oxalate and calcium phosphate were made of amorphous material and were dispersed in a matrix containing amorphous, fibrillar, and membranous substances. Malachite green positive material was seen to be associated with the ghosts, as well as with the membranous and fibrillar components of the organic matrix. Calcium oxalate and calcium-phosphate crystals, induced in human urine in vitro were also found to be associated with an organic matrix containing lipids and proteins. It is suggested that the intimate association between crystals and lipids is a result of the involvement of cellular membranes in the nucleation of these crystals.