We have isolated the gene encoding mouse placental lactogen-I and characterized the promoter region of this gene by transient and stable transfection. Promoter sequences extending 274 basepairs (bp) up-stream from the start site of transcription contain all of the elements necessary for maximal expression upon transient transfection into the rat choriocarcinoma Rcho-1 cell line; these Rcho-1 cultures contain both proliferative trophoblast stem cells and terminally differentiated trophoblast giant cells. In stably transfected cell lines, expression from this promoter increases as the percentage of differentiated cells in the culture increases. In contrast to these results in trophoblast cells, the 274-bp promoter as well as a promoter region extending 2700 bp up-stream of the transcriptional start site are unable to drive transcription in a variety of other cell types. Mutational and protein binding analyses indicate that two AP-1 sites are required for maximal expression in Rcho-1 cells, and that the composition of the AP-1 transcription factor may vary as differentiation in the cell culture increases. In addition to these two AP-1 sites, at least one other element appears to be critical for promoter activity in trophoblast cells.