Miniplasmids (pKN402 and pKN410) were isolated from runaway-replication mutants of plasmid R1. At 30 degrees C these miniplasmids are present in 20--50 copies per cell of Escherichia coli, whereas at temperatures above 35 degrees C the plasmids replicate without copy number control during 2--3 h. At the end of this period plasmid DNA amounts to about 75% of the total DNA. During the gene amplification, growth and protein synthesis continue at normal rate leading to a drastic amplification of plasmid gene products. Plasmids pKN402 (4.6 Md) and pKN410 (10 Md) have single restriction sites for restriction endonucleases EcoRI and HindIII; in addition plamid pKN410 has a single BamHI site and carries ampicillin resistance. The plasmids can therefore be used as cloning vectors. Several genes were cloned into these vectors using the EcoRI sites; chromosomal as well as plasmid-coded beta-lactamase was found to be amplified up to 400-fold after thermal induction of the runaway replication. Vectors of this temperature-dependent class will be useful in the production of large quantities of genes and gene products. These plasmids have lost their mobilization capacity. Runaway replication is lethal to the host bacteria in rich media. These two properties contribute to the safe use of the plasmids as cloning vehicles.