The effects of interleukin-6 (IL-6), a cytokine recently found to be secreted by monocytes and macrophages, on c-myc expression and proliferation of cultured vascular smooth muscle cells (VSMC) were investigated. Treatment with IL-6 caused rapid increase in the c-myc mRNA level of VSMC. It also stimulated DNA synthesis and proliferation of the cells significantly and dose-dependently at concentrations of more than 10 U/ml. These results suggest that IL-6 may be important in the proliferation of VSMC, which is a key event in the development of arteriosclerosis, as a factor mediating immune cell-VSMC interaction.