The effects of dimethyl sulphoxide have been investigated on differentiation in human rhabdomyosarcoma cell lines obtained from typically malignant, poorly differentiated tumours. The expression of cell differentiation marker proteins (desmin and vimentin) was assessed in cell lines A-204, A-673 and RD, and the modifications in expression after 3, 8 and 24 h of induction with 1.25% dimethyl sulphoxide were recorded. Protein expression in both the cytoplasm and cytoskeleton was significantly altered by treatments lasting 8 and 24 h, the most noteworthy changes being increased desmin and decreased vimentin expression. The results clearly indicate that dimethyl sulphoxide induced changes typical of differentiation in rhabdomyosarcoma cell lines A-673 and RD; less marked changes were observed in line A-204.