A new plasmid vector, pNS1, is described that allows positive selection for bacterial transformants carrying recombinant plasmids. It is a derivative of pBR327, and it includes a regulatory region from the lambdoid phage 434. The expression of the TcR gene of pNS1 is under the control of the ORpR operator-promoter of phage 434, which is regulated by the repressor gene cI. The cloning sites of pNS1 (StuI, NdeI, HpaI, HindIII, AsuII and EcoRI) are situated within cI; hence insertion of foreign DNA into these sites causes depressed expression of the TcR gene from pR thus conferring the TcR phenotype on the harboring Escherichia coli strain. The use of pNS1 is facilitated by the presence of another selectable marker, ApR, its small size, and its known nucleotide sequence; no special host strain is required.