The gene for mouse placental lactogen II (mPL-II) has been isolated and characterized. This gene contains five exons, with a transcription start site 59 nucleotides upstream of the translation initiation ATG. Introduction of a DNA construct containing 2.7 kilobases of sequence upstream of the mPL-II transcription initiation site directed the synthesis of a linked coding region for the simian virus 40 large and small tumor antigens in placental trophoblast giant cells of transgenic mice. The pattern of simian virus 40 transgene expression in the placenta was indistinguishable from that of the endogenous mPL-II gene. In contrast, the first 569 base pairs upstream of the transcription start site proved insufficient to direct placental expression. Thus, one or more elements required for placental trophoblast giant cell expression have been localized to a region between -2700 and -569 of the mPL-II gene.