A non-viral gene therapy vector, pcDNA3-EPO, was constructed by subcloning erythropoietin (EPO) cDNA into plasmid pcDNA3. After liposome-mediated transfection of the NIH 3T3 cells in vitro, EPO expression in the culture medium was detected by ELISA and amounted to 1.25 +/- 0.3 IU ml(-1). The biological activity of this EPO in the medium was detected after intramuscular injection of BALB/c mice. PCR of genomic DNA and RT-PCR of total RNA also confirmed that the plasmid pcDNA3-EPO had been transfected into the cells. A pool of pcDNA3-EPO transfectants, which stably expressed EPO, was obtained by G418 selection. When pcDNA3-EPO was combined into liposomes and intramuscularly injected into BALB/c mice, the reticulocyte ratio in the positive mice was three times higher than that in the control mice. In vivo expression was maintained in mice for at least one month.