The hepatic 35 kD nucleoprotein participates in transcriptional regulation of the haptoglobin gene during the development of rat liver. The same molecular mass and trans-acting role of the above protein as the active isoform of C/EBPbeta transcription factor, made homology between them possible. We could detect C/EBPbeta in control hepatic nuclear extracts not earlier than in the second week after the birth. However, the acute-phase reaction induced the expression of 35 kD-C/EBPbeta at day 7 of postnatal development suggesting that the above trans-active nucleoprotein displays the structural and functional characteristics of C/EBPbeta isoform from that day on.