In order to identify nucleoproteins involved in transcriptional regulation of the haptoglobin (Hp) gene, fetal rat livers of dams exposed to inflammation on day 19 of pregnancy were used. Previously observed acute phase-dependent elevation of Hp gene transcriptional activity in prenatal liver was accompanied by increased binding affinities of several fetal soluble nucleoproteins and the hormone response element (RE) of the Hp gene (-170/-56). One of these proteins, a hepatic nucleoprotein of 53kDa, was identified by Western blotting analysis as a protein within the same molecular mass and epitopes as transcription factor p53. Also, in vitro phosphorylation experiments revealed that the examined fetal nucleoprotein could be liable to the same phosphorylative post-translational modification as p53. The obtained results suggest that the fetal 53kDa-nucleoprotein could be a homologue of transcription factor p53, participating in the transcriptional modulation of the Hp gene throughout prenatal hepatic development.