IL-6 signal transduction occurs when the liganded interleukin-6 receptor (IL-6R) interacts with glycoprotein (gp) 130. We hypothesized that synthetic peptides modeled from the extramembranous domain of the IL-6R may interfere with the IL-6-induced reaction between IL-6R and gp130 and may serve to elucidate the initial steps in IL-6 signal transduction. The capacity of such peptides to modulate two different IL-6 functions was evaluated: 1) IL-6-dependent B9 cell mitogenesis, and 2) IL-6-induced acute phase protein synthesis in HepG2 cells. A synthetic peptide, 249Y16T264, corresponding to residues 249-264, inhibited IL-6-dependent B9 proliferation and IL-6-induced acute phase protein up-regulation in HepG2 cells. Other peptides modeled from different regions of the IL-6R were not inhibitory. 249Y16T264 did not inhibit IL-6-independent HepG2 cell proliferation or total cellular protein synthesis. The inhibitory effect was reversible, indicating that the peptide was not cytotoxic. 249Y16T264 did not inhibit 125I-IL-6 binding in U266 cells. Delineation of this domain identified 249Y10R258 as the minimum effective sequence capable of inhibiting fibrinogen synthesis. Amino acid substitutions in 249Y10R258 obliterated the inhibitory effect on fibrinogen synthesis. In conclusion, a region of the extramembranous domain of the IL-6R has been identified that is involved in the regulation of IL-6 signal transmission. A synthetic peptide representing this region inhibits IL-6-dependent B9 cell mitogenesis and IL-6-stimulated acute phase response in HepG2 cells without affecting ligand binding.