An efficient expression system for the previously only weakly expressed thermophilic lipase BTL2 (Bacillus thermocatenulatus lipase 2) was developed for the production of large amounts of lipase in Escherichia coli. Therefore, the gene was subcloned in the pCYTEXP1 (pT1) expression vector downstream of the temperature-inducible lambda promoter P-L. Three different expression vectors were constructed : (i) pT1-BTL2 containing the mature lipase gene, (ii) pT1-preBTL2 containing the prelipase gene and (iii) pT1-OmpABTL2 containing the mature lipase gene fused to the signal peptide of the OmpA protein, the major outer membrane protein of E. coli. With pT1-BTL2 and pT1-preBTL2, comparable expression levels of 7000-9000 U/g cells were obtained independently of the E. coli host. In contrast, with E. coli JM105 harbouring pT1-OmpABTL2, 660000 soluble lipase U/g cells was produced, whereas, with E. coli DH5 alpha and BL321, production levels of 30000 U/g cells were achieved. However, most of the lipase remained insoluble but active after cell breakage because of the unprocessed OmpA signal peptide. A simple cholate extraction followed by proteinase K cleavage and ultrafiltration allowed the isolation of 1.15 x 10(6) units of 90% pure mature lipase/wet cells.