Small low-copy-number plasmid vectors were constructed by in vitro and in vivo recombinant DNA techniques. pLG338 and pLG339 are derived from pSC105, have a copy number of six to eight per chromosome, and carry genes conferring resistance to tetracycline and kanamycin. pLG338 (7.3 kb) has unique restriction endonuclease sites for BamHI, SalI, HincII, SmaI, XhoI, EcoRI and KpnI, the first five lying within a drug resistance gene. pLG339 (6.2 kb) lacks the KpnI site, but has unique SphI and PvuII sites. These versatile vectors should be useful for cloning many genes coding for membrane and regulatory proteins which cannot be cloned into high-copy-number plasmids.