Members of the suppressor of cytokine signaling (SOCS) family are potentially key physiological negative regulators of interleukin-6 (IL-6) signaling. To examine whether SOCS3 is involved in regulating this signaling, we have used conditional gene targeting to generate mice lacking Socs3 in the liver or in macrophages. We show that Socs3 deficiency results in prolonged activation of signal transducer and activator of transcription 1 (STAT1) and STAT3 after IL-6 stimulation but normal activation of STAT1 after stimulation with interferon-(IFN-). Conversely, IL-6-induced STAT activation is normal in Socs1-deficient cells, whereas STAT1 activation induced by IFN- is prolonged. Microarray analysis shows that the pattern of gene expression induced by IL-6 in Socs3-deficient livers mimics that induced by IFN-. Our data indicate that SOCS3 and SOCS1 have reciprocal functions in IL-6 and IFN- regulation and imply that SOCS3 has a role in preventing IFN--like responses in cells stimulated by IL-6.