Egr-1 is a ubiquitous immediate-early gene whose expression is induced by a wide range of different stimuli. A requirement for egr-1 expression has been demonstrated in pathways leading to both proliferation and differentiation, suggesting that egr-1 is a critical intermediary in determining the long-term cellular response to a stimulus. To determine how egr-1 coordinates a cellular response to receptor-mediated stimulation, we have developed a transient cotransfection assay to map functional domains in the EGR1 protein. We localized an activation domain to a serine/threonine/proline-rich region between amino acids 174 and 270. Using this information, we designed a mutant that lacks this activation domain, but retains the DNA-binding domain. When cotransfected into fibroblasts with an EGR1-dependent reporter, this mutant inhibited the transcriptional activity of both endogenous EGR1, as well as exogenously expressed, wild-type EGR1 protein. These data demonstrate that the activation domain of EGR1 is critical for the activity of the protein, and that a mutant lacking this domain can dominantly inhibit wild-type EGR1 function.