Plasmid expression vector using the temperature-regulated promoter P'R of bacteriophage lambda is described. The vector carries a combination of two regions of lambda cI857indgenome, that contain: 1) gene cI and promoter PR, and 2) gene Q and promoter P'R. Transcription or gene Q is initiated at promoter PR, which is controlled by cI857 repressor. The Q gene product acts as a positive regulator of RNA synthesis from P'R. At 37 degrees C, sufficient amounts of protein Q are synthesised to initiate the expression of the cloned gene from P'R. Inactivation of Q gene (by elimination of a single NcoI site) results in the loss of P'R expression activity in the vector. E. coli beta-galactosidase gene (lacZ) and human leukocyte interferon alpha 2 gene (ifn alpha 2) were cloned into a single EcoRI cleavage site under the control of P'R. These constructs express high levels of beta-galactosidase and interferon alpha 2 in E. coli at 37 degrees C.