A broad panel of agents including serum, interleukin-1, double-stranded RNA, and platelet-derived growth factor (PDGF) stimulate transcription of the "slow" immediate-early gene MCP-1. These disparate inducers act through a tight cluster of regulatory elements in the distal 5'-flanking sequences of the MCP-1 gene. We describe a 22-base element in this cluster which, in single copy, confers PDGF-inducibility to a tagged MCP-1 reporter gene. In mobility shift assays, the element binds a PDGF-activated form of NF-kappaB, and a 90-kDa protein (p90) which binds constitutively. Antibody supershift and UV cross-linking experiments indicate that the PDGF-activated NF-kappaB species is a Rel A homodimer. The DNA binding form of p90 is a nuclear-restricted serine/threonine phosphoprotein. Mutagenesis of the 22-base element shows that the NF-kappaB and p90 binding sites overlap, but binding of the two species is mutually independent. Both sites, however, are required for optimum PDGF induction of MCP-1. Therefore, p90 appears to be a coactivator with NF-kappaB in PDGF-mediated induction of MCP-1.