The studies reported in this paper were designed to test the hypothesis that a cis element located in the 3' UTR of manganese superoxide dismutase (MnSOD) RNA, designated MnSOD-response element (MnSOD-RE), is a translational enhancer in vivo. NIH/3T3 cells were transfected with a posttranscriptional reporter construct in which MnSOD-RE was placed 3' of the coding region of chloramphenicol acetyltransferase (CAT); this construct is designated CAT-RMS. Transient transfection of CAT-RMS did not change the concentration of CAT mRNA but increased CAT activity by similar to 400% compared to a control construct, CAT-V, which contains approximately the same size of nonMnSOD 3' UTR sequence. Transfection of CAT-RMS had no effect on endogenous MnSOD protein, mRNA, or MnSOD RNA-binding protein activity. Because of its ability to increase translation of a heterologous RNA MnSOD-RE may be useful in designing expression vectors for in vitro expression systems and in vivo gene therapy.