We developed a novel promoter system, designated SRα, which is composed of the simian virus 40 (SV40) early promoter and the R segment and part of the U5 sequence (R-U5′) of the long terminal repeat of human T-cell leukemia virus type 1. The R-U5′ sequence stimulated chloramphenicol acetyltransferase (CAT) gene expression only when placed immediately downstream of the SV40 early promoter in the sense orientation. The SRα expression system was 1 or 2 orders of magnitude more active than the SV40 early promoter in a wide variety of cell types, including fibroblasts and lymphoid cells, and was capable of promoting a high level of expression of various lymphokine cDNAs. These features of the SRα promoter were incorporated into the pcD-cDNA expression cloning vector originally developed by Okayama and Berg.