Ganglioside distribution was studied in peripheral nerves of normal controls and those of Trembler mutant mouse with defect in Schwann cell differentiation and myelination. Neuraminic acid content was considerably decreased in the mutant. Ganglioside distribution as evaluated by densitometry of resorcinol positive spots on thin-layer chromatography revealed a major peak for GD1a in normal controls. In the mutant, the relative proportion was modified with qualitative modifications in the GD1a area and a tremendous increase in GM3 content. The relation with the intense Schwann cell proliferation observed in the mutant is discussed.