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Maitotoxin (MTX) at 0.3 nM elicited a 10−20-fold increase in the level of Ca²⁺ influx in rat glioma C6 cells. At higher doses (3−30 nM), MTX induced marked Ca²⁺ influx in human erythrocyte ghosts when monitored with the fluorescent dye Fura-2. Although the ghosts were not as susceptible to MTX as intact erythrocytes or other cell lines, Fura-2 experiments under various conditions suggested that the MTX-induced entry of ions into the ghosts was mediated by a mechanism similar to that reported for cells or tissues. These ghosts are the simplest system known to be sensitive to MTX and thus may be suitable for research on the direct action of MTX. Gangliosides GM1 and GM3, glycosphingolipids which have a sialic acid residue, strongly inhibited MTX-induced Ca²⁺ influx in C6 cells, while the inhibitory action by asialo-GM1, which lacks a sialic acid residue, was somewhat weaker. Their inhibitory potencies were in the following order:  GM1 (IC50 ~ 2 μM) > GM3 (IC50 ~ 5 μM) > asialo-GM1 (IC50 ~ 20 μM). GM1 (3 μM) completely blocked MTX (30 nM)-induced Ca²⁺ influx in human erythrocyte ghosts. When C6 cells were pretreated with tunicamycin, an antibiotic which inhibits N-linked glycosylation, or concanavalin A, a lectin which exhibits a high affinity for cell-surface oligosaccharides, MTX-induced Ca²⁺ influx was significantly potentiated. This suggests that removal of oligosaccharides from the cell surface by tunicamycin or capping of sugar chains on plasma membranes by concanavalin A can potentiate the action of MTX.