The effects of low doses of dimethyl sulfoxide (DMSO) on the growth and morphology of tumour NGUK-1 strain cells from neurinoma were studied. DMSO produced a dose-dependent reduction in the proliferative capacity. 3% and 5% DMSO concentration inhibited the mitotic activity in the culture and the entry of cells into S-phase of the cell cycle, which was demonstrated by decreased mitotic and thymidine-labelling index. Electron microscopic studies at these DMSO concentrations have revealed large cisterns of rough endoplasmic reticulum with electron dense fine granules. Nucleoli had a spongy structure. DMSO induced stimulation of protein synthesis in cells. At greater DMSO concentrations almost all the cells died. At a 1% concentration DMSO had no effect on cellular morphology and proliferation of in vitro propagated tumour cells.