The present invention is directed to a novel naturally occurring human cytokine that is comprised of a heterodimer of interleukin-17 and interleukin-17F designated herein as interleukin 17A/F (IL-17A/F). Also provided herein are vectors and host cells comprising those nucleic acid sequences, chimeric polypeptide molecules comprising the polypeptides of the present invention fused to heterologous polypeptide sequences, specific antibodies which bind to the polypeptides of the present invention and to methods for producing the polypeptides of the present invention. Further provided herein are methods for treating degenerative cartilaginous disorders and other inflammatory diseases.