Abstract: Plasmids useful as cloning vectors carrying an inserted regulatable promoter, transcription from the promoter regulating plasmid replication. Increased transcription leads to a substantially increased or uncontrolled plasmid copy number when host microorganisms containing the plasmid are cultivated under conditions securing such increased transcription. The regulatable promoter is preferably .lambda. P.sub.R, the activity of which is controlled by a temperature-sensitive .lambda. cl repressor. To obtain gene products of the plasmids, host microorganisms to which such plasmids have been transformed are desirably cultivated at about 30.degree. C. to secure a constant, low plasmid copy number during the seeding and multiplication stages of the microorganisms, after which the temperature is preferably shifted to about 36.degree.-42.degree. C. to obtain an uncontrolled plasmid copy number and an amplified amount of gene product.