We have observed significant cell lysis upon temperature up-shift of recombinant Escherichia coli cultures harboring CI857repressed lambda-based expression vectors. This event, that becomes evident about 30^40 min after the heat shock, takes place when using the lambda promoter system in Ind 3 lysogenic strains, but not in others commonly employed for recombinant gene expression. These results strongly suggest that the thermosensitive CI857 repressor, encoded by the expression vector, competes with CI Ind 3 molecules for binding to the prophage operator region, allowing for expression of lytic genes from the integrated Ind 3 viral genome upon temperature up-shift. Transcription of viral lytic genes does not include unspecific expression of a reporter sulA: :lacZ gene fusion carried in the prophage genome. These results prompt, however, to carefully evaluate the limitations of expression systems based on p L /p R -CI857 in bacterial strains modified through lambda Ind 3 gene transfer vehicles. ß