The human interleukin-6 receptor (IL-6R) was differentially expressed on IL-6-dependent (U266 and SKO-007) and -independent (RPMI8226) myeloma cells as well as melanoma cells (A375-C6) that are growth-inhibited by IL-6. U266 and SKO-007 cells expressed four distinct IL-6R complexes (molecular masses of 100, 120, 145, and 165 kDa) as revealed by affinity cross-linking of iodinated IL-6. RPMI8226 and A375-C6 cells primarily expressed the 165-kDa complex relative to the others. Immunoprecipitation and antibody competition studies showed that the 100- and 120-kDa complexes contained the gp80 subunit, whereas the 145- and 165-kDa complexes contained the gp130 subunit of the IL-6R. Assaying solubilized U266 plasma membrane proteins by affinity cross-linking or ligand blotting revealed that only gp80 bound IL-6 specifically. Induction of an IL-6 response was associated with ligand-induced down-regulation of gp130 and was inhibited by neutralizing anti-IL-6 antibodies. Furthermore, the relative ratios of gp80 to gp130 determined the binding kinetics of the IL-6R, yielding high- and low-affinity binding sites by Scatchard plots. Our data imply that distinct IL-6 bioactivities are based upon the differential expression and regulation by IL-6 of its ligand-binding (gp80) and signal-transducing (gp130) receptor subunits.