The effects of interleukin-1β (IL-β) and interleukin-6 (IL-6) on proliferation of cultured vascular smooth muscle cells (VSMCs) were investigated. Treatment with IL-6 caused a rapid increase in the c-myc mRNA level, and resulted in increases in DNA synthesis and cell number. IL-1β stimulated the DNA synthesis of the cells. EGF showed synergistic and PDGF or IL-1β showed additive effects with IL-6 on the DNA synthesis. These results suggest that IL-6, independently of IL-1β, may be important in the proliferation of VSMCs.