C/EBPβ is a member of the CCAAT/enhancer binding protein family of transcription factors and has been shown to be a critical transcriptional regulator of various proinflammatory genes, including IL-6 and MCP-1. To examine the roles of the C/EBPβ transactivation and regulatory domains in LPS-induced MCP-1 and IL-6 expression, we expressed various N-terminal truncations and deletions of C/EBPβ in P388 murine B lymphoblasts, which lack endogenous C/EBPβ expression and are normally unresponsive to LPS for expression of IL-6 and MCP-1. Unexpectedly, a region between amino acids 105 and 212 of C/EBPβ that includes regulatory domains 1 and 2 facilitates C/EBPβ activation of IL-6 expression, while having an inhibitory effect on MCP-1 expression. Thus, this region can mediate promoter-specific effects on cytokine and chemokine gene transcription. LIP, the naturally occurring truncated form of C/EBPβ, largely retains these regulatory domains and stimulates IL-6 but not MCP-1 transcription.