Glycosphingolipids, labeled with the fluorescent probe lissamine rhodamine were administered to skin fibroblasts in culture and were hydrolyzed in the intact cells to the corresponding lissamine rhodamine ceramide (N-acylsphingosine). This fluorescent ceramide was converted in the intact cells to the corresponding sphingomyelin which was secreted into the culture medium. In comparison, ceramide is not formed in cells derived from patients with lipid storage diseases, because of deficiencies in lysosomal glycolipid hydrolases. Consequently, fluorescent sphingomyelin was absent from the culture medium or present in considerably reduced quantities. This provided a procedure for diagnosing lipidoses, by analyzing the lissamine rhodamine sphingomyelin content in the culture medium, while maintaining the cells intact.