D-Glucosamine is toxic to several malignant cell lines and in vivo tumors at concentrations that have little effect upon normal host tissues. Evidence is presented to support the hypothesis that cellular membranes may be the primary targets of glucosamine's tumoricidal activity. Treatment of rat C6 glioma cells with a cytotoxic concentration of glucosamine (20 mM) caused fragmentation of rough endoplasmic reticulum, proliferation of Golgi complexes, evagination of outer nuclear and mitochondrial membranes, and the accumulation of membranous vacuoles and lipid droplets in the cytoplasm. These changes were detected within the first 3 hr after treatment of cultures with glucosamine and became increasingly severe until cell lysis occurred between 24 and 48 hr of treatment. The cytotoxicity of glucosamine was potentiated by the local anesthetic lidocaine, and by other membrane-active drugs, at concentrations that were growth inhibitory but nonlytic. Most of these drugs possessed local anesthetic activity and inhibited glioma sterol synthesis. Within the same period of time required for ultrastructural changes in cellular membranes, glucosamine inhibited the incorporation of [2-(14)C]acetate into sterols and into an unidentified 400-dalton lipid that migrated close to sterols on thin-layer chromatograms. This inhibition was potentiated by lidocaine and increased over the same range of D-glucosamine concentrations that led to increased cell toxicity after a 48-hr treatment. These findings suggest that the effects of glucosamine upon cellular membranes may be central to its tumoricidal activity and that glucosamine, in combination with membrane-active drugs, may be useful in the treatment of certain types of tumors, particularly those of the central nervous system.