A promoter vector pACPR33 for Escherichia coli based on the promotorless ampicillin-resistance gene from pBR322 has been constructed, The promoter of the ampicillin-resistance gene was deleted and replaced by a suitable multiple cloning site, Molecular cloning of promoters into the polylinker resulted in activation of the ampicillin resistance in E. coli. The plasmid contains a functional origin of DNA replication and a tetracycline resistance gene for E. coli, and a chloramphenicol resistance gene for S. aureus. The vector permitted direct detection of promoter activity, especially strong promoters, by easy iodometric determination of beta-lactamase activity in liquid or solid media.