HNF1 is a liver-specific transcription factor that plays the dominant role in determining the cell type-specific in vitro transcription of the albumin gene. Here we report the purification and preliminary characterization of HNF1. HNF1 appears to be heavily glycosylated since it is retained on a wheat germ agglutinin-agarose column and can be eluted from it with N-acetylglucosamine, a property not observed with other factors binding to the albumin promoter. Using in vitro transcription assays we demonstrate that purified HNF1 strongly stimulates albumin promoter activity in spleen nuclear extracts, which are devoid of this factor. Likewise, an artificial promoter consisting of two HNF1 recognition sites in front of a TATA motif is strongly activated by HNF1 in such extracts. In addition to stimulating transcription directly by binding to its cognate site, HNF1 may further enhance albumin promoter activity by interacting cooperatively with other trans-acting factors.