2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a highly toxic compound that has recently attracted much attention as an environmental contaminant, elicits a variety of toxic responses. Most, if not all, of the toxic effects of TCDD are thought to result from alteration of gene expression. TCDD acts through both transcriptional and posttranscriptional mechanisms to alter gene expression of many genes. Transforming growth factor (TGF)-alpha and urokinase-type plasminogen activator (uPA) are examples of the genes up-regulated posttranscriptionally by TCDD by mRNA stabilization. While effects of TCDD on transcription have been extensively studied, the molecular mechanisms underlying the TCDD-induced changes in mRNA stability are poorly understood. In this study, we investigated the trans-acting factors involved in TCDD-dependent mRNA stabilization. UV-crosslinking study showed that a liver cytoplasmic protein of 50 kDa (p50) selectively recognized the 3' UTR of the uPA mRNA in a TCDD-dependent manner. We also showed that the activation of p50 by TCDD is mediated through a protein phosphorylation cascade but not via de novo protein synthesis. This is the first study to show the presence of the TCDD-dependent RNA binding activity which may be involved in TCDD-dependent stabilization of mRNA.