Transcriptional regulation and binding interactions between soluble nucleoproteins and the hormone response elements (REs) of the rat haptoglobin (Hp) and alpha 2-macroglobulin (alpha 2MG) genes was examined in the livers of rats during the acute-phase reaction. Our results demonstrate that the elevation of the Hp and alpha 2MG genes' transcription rates in acute-phase liver relies essentially on an increase in the binding-affinity of pre-existing soluble nucleoproteins, enhancing their capability to bind the examined cis-regulatory elements. The 35kD nucleoprotein that displayed an acute-phase inducible affinity to bind hormone REs of rat Hp and alpha 2MG genes, was identified as a C/EBP beta isoform.