The human genes coding for growth hormone (hGH) and placental lactogen (choriosomatomammotropic hormone [hCS]) are clustered on chromosome 17 in the following order: 5' hGH-N hCS-L hCS-A hGH-V hCS-B 3'. So far, a single placenta-specific enhancer has been identified in the locus, 2 kb downstream from the hCS-B gene, and shown to comprise one in vitro binding site for a nuclear protein. We here provide evidence that the hCS-B enhancer is more complex: (i) protection against DNase I digestion in the 3' flanking region of the hCS-B gene reveals four binding sites (DF-1, DF-2, DF-3, and DF-4) for nuclear proteins from either placental or HeLa cells, and (ii) placenta-specific enhancer activity can be fully exerted in transient expression experiments by a 126-bp fragment comprising the DF-3 and DF-4 protein-binding sites. By dissecting this region, we show that enhancer activity is mediated by a synergy between DF-3 and DF-4. Competitions with various oligonucleotides in footprinting and gel retardation experiments indicate that the same protein or set of proteins, different in HeLa and placenta cell nuclei, interacts with sites DF-2, DF-3, and DF-4. We also studied the regions of the hCS-L and hCS-A genes which are highly similar to the hCS-B enhancer. Although they each present the same four protein-binding sites, they exhibit only minor enhancer activity.