LBP-1b, LBP-9, and LBP-32/MGR (LBP proteins) are transcriptional factors that regulate the expression of the P450 side-chain cleavage enzyme (P450scc) in the human placenta. Placenta contains at least two types of trophoblasts: cytotrophoblasts and syncytiotrophoblasts. P450scc has been detected in syncytiotrophoblasts through all stages of pregnancy. The expression of LBP proteins in different placental stages is unknown. We isolated total RNA from cytotrophoblasts and syncytiotrophoblasts of both first-trimester and full-term human placenta. The mRNA expressions of LBP proteins were detected only in the syncytiotrophoblasts from both first-trimester and full-term placenta. To determine the regulation among LBP proteins, we isolated the 5'-flanking region of one of the LBP proteins (LBP-32/MGR). After determining the transcriptional initiation site by a primer extension assay, we isolated and tested the activity of different lengths of the LBP-32/MGR promoter. A core promoter region for LBP-32/MGR extending from -639 to -184 bp was used to determine the interaction among LBP proteins. We cotransfected LBP-1b and LBP-9 with the LBP-32/MGR promoter and found that both stimulated LBP-32/MGR promoter activity. Our data suggested that an interaction exists among these transcriptional factors at the transcriptional level and provided us with information in our basic understanding of P450scc regulation.