A runaway vector for mammalian cells was constructed from the simian virus 40 (SV40) genome with a temperature-sensitive mutation of the large T antigen and bacterial neo(r) gene. Replication of this plasmid was repressed above 39(degrees)C and vigorous DNA propagation was observed below 33 degrees C in simian CV-1 cells. The human erythropoietin gene was inserted downstream of the SV40 late promoter of the plasmid and the recombinant plasmid was introduced into CV-1 cells. By a temperature shift from 37 to 33(degrees)C, the plasmid copy number increased from 5 x 10(2) to 5 x 10(3) copies per cell and the specific production rate of erythropoietin increased more than ten-fold. The bacterial derived sequences such as the neo(r) gene and vector pUC sequences were prone to delete but the main body of the recombinant plasmid such as SV40 and the erythropoietin-coding sequences were stably maintained at either 33 or 37 degrees C.