Improved expression vectors have been constructed which are derived from runaway-replication mutants of plasmid R1 and carry the strong leftward promoter (pL) of bacteriophage lambda. The activity of this promoter is controlled by a temperature-sensitive repressor, product of the phage gene cI cloned on a compatible plasmid. Heat induction leads to amplification of the plasmid copy number and at the same time turns on the promoter. At a short distance downstream from the promoter, unique EcoRI, BamHI, XbaI and HindIII sites are present. This system was used for high level expression of the T4 DNA-ligase gene; 3 h after induction the ligase amounted to about 20% of total cellular protein.