The sequences upstream of a proliferin gene have been isolated, linked to a reporter gene, and transfected into mouse cell cultures. In low serum concentrations, transcription from the transfected DNA is very weak; transcriptional activity is induced 20- to 40-fold in transfected cultures grown in high serum concentrations. Initiation of transcription occurs at the same site in the transfected DNA as in endogenous proliferin genes expressed in placental tissue and in cell cultures. Sequences within 578 nucleotides upstream of this initiation site are sufficient for complete serum-inducible expression, but deletion of the upstream sequences to within 211 nucleotides of the start site abolishes promoter activity. In contrast, the upstream region from a second proliferin gene is only weakly inducible in transfected cell cultures, even though these two promoter regions share 97% nucleotide sequence homology.