To identify the promoter sequence(s) of the CG beta gene, genomic fragments derived from a cosmid containing the CG beta gene family were transfected into mouse Y1 adrenal cortical cells. Using this system, we showed that the CG beta genes 5, 3, and 8 have functional promoters, the basal element of which in the case of CG beta 5, was within 78 base pairs 5' ward from the CAP site. The size of the CG beta transcripts and identity of the transcription start site was the same for CG beta mRNA synthesized in Y1 cells as in first trimester placenta. The promoter region identified by this system was also capable of driving the chloramphenicol acetyltransferase gene when transfected stably into choriocarcinoma cells. Chloramphenicol acetyltransferase constructs bearing variable length of 5'-flanking sequences from CG beta 5 and transfected into trophoblast cells suggest the presence of regulatory sequences within 700 base pairs from the CAP site. The information obtained here provide a foundation for studies of analyzing trans-acting placental and pituitary proteins to the defined CG beta promoter region.