Polypeptide chain elongation factor la (EF-la) is an eukaryotic counterpart of E. coli EF-Tu which promotes the GTP-dependent binding of an aminoacyl-tRNA to ribosomes. EF-lcx is one of the most abundant proteins in eukaryotic cells, and expressed in almost all kinds of mammalian cells. Recently, we have isolated human chromosomal gene coding for EF-la, and shown that the promoter of EF-la chromosomal gene very efficiently stimulates the in vitro transcription (1). In this report, we have constructed a powerful mammalian expression vector, pEF-BOS, using the promoter of human EF-la chromosomal gene. As shown in Fig. 1, pEF-BOS carries the SV40 replication origin (311 bp of EcoRH G fragment), the promoter region of human EF-la chromosomal gene (1.2 kb), the stuffer fragment (450 bp) from CDM8 vector (2) and poly (A) adenylation signal from human G-CSF cDNA (700 bp Eco8lI-EcoRI DNA fragment)(3) in HindIH-EcoRI site of pUCl 19. The promoter region of EF-lc gene is from nucleotide position 373 to 1561 (1) which includes 203 bp 5'flankingregion, 33 bp first exon, 943 bp first intron and 10 bp of the part of the second exon located at 20bp upstream of the ATG initiation codon. The size of pEF-BOS is 5.8 kb, and the cDNA to be expressed can be inserted at BstXIsite using BstXI adapter, or XbaI site using XbaI linker. Human G-CSF cDNA (4) was inserted into BstXI site ofpEF-BOS or CDM8, or into BamHI site of pKCR vector containing SV40 early promoter (5). As shown in Table 1, when these plasmids were transfected into COS cells by DEAE-dextran/chloroquine method, the construct in pEF-BOS has directed the synthesis of human G-CSF about 20 times more efficiently than the construct in CDM8, and 50-200 times more efficiently than the construct in pKCR. In addition, when E. coli chloramphenicol acetyltransferase (CAT) gene was inserted into pEF-BOS, the CAT activities observed with pEF-BOS-CAT were 1.5-50 times higher than that of pSV2-CAT or pRSV-CAT after transfection into various cell lines including murine L929, human HeLa, CHU-2 and simian COS cells (Table 2). The pEF-BOS vector, therefore, will be used to produce a large amount of growth factors and proteins in mammalian cells, to express a high level of anti-sense RNA. Furthermore, the pEF-BOS-CAT will be an ideal positive control for CAT assay in various cell types.