Lipid staining, using oil red 0 and Sudan black B stain, was carried out on frozen sections of formalin-fixed tumour tissue from 142 cases of malignant lymphoma. Most cases of Burkitt's tumour contained abundant coarse lipid droplets both within the cytoplasm of the lymphoid cells and within non-neoplastic histiocytes scattered throughout the tumour. Lymphocytic lymphomas contained little stainable fat. Histiocytic and stem-cell lymphomas were almost equally divided between those containing abundant lipid droplets and those devoid of lipid. It is suggested that the demonstration of abundant cytoplasmic lipid droplets may assist in the diagnosis of Burkitt's tumour although this feature is not in itself diagnostic. The significance of the lipid droplets in the cells of Burkitt's tumour is discussed in relation to the histogenesis of the tumour.