Cytotoxic agents used in cancer therapy may induce differentiation in tumour cells with no proliferative potential. However, chemotherapy can also induce multidrug resistance, a formidable obstacle to the successful treatment of tumours. Both events were recently shown to occur in a rhabdomyosarcoma cell line (RD-DAC) resistant to actinomycin D, a drug of choice in the treatment of these tumours. To analyse this connection, cell line R D cultures were investigated with progressive concentrations of actinomycin D and it was shown that a minimum dose (1.2 x l o p 6 mM) of the drug was necessary to increase mdv 1 mRNA in RD-DAC. The mechanism of mdv 1 overexpression was an increase in the number of copies of the mdv 1 gene, although the mRNA levels were not correlated with rndv 1 amplification. Drug resistance mediated by mdv I overexpression coincided with the development of myogenic differentiation in RD-DAC and with a decrease in c-myc mRNA levels, whereas levels of N-myc mRNA showed no modulation. These findings suggest that factors implicated in cell proliferation and differentiation, such as c-myc, may be responsible for the control of genes related to the development of multidrug resistance in rhabdomyosarcomas. Modulation of these factors may determine the sensitivity of rhabdomyosarcoma cells to drugs and may play an important role in triggering the differentiation programme found in these resistant rhabdomyosarcoma cells.