The failed attempt to express HIV-gp41 inE. coli led to the investigation of HIV-gp41 segments, which is responsible for the toxicity toE. coli cells. A series of deletion mutants containing different regions ofgp41 gene were constructed and expressed inE. coli BL21(DE3) strain. After IPTG induction, the high mortality of host bacteria was observed in host bacteria carrying the deletion mutants ofgp41 gene except for those transformed with pET-HN2; coordinately, the mRNA transcripts of thegp41 was rapidly decreased; and the release of [3H]uridine increased upon induction. All these data suggested that GP41 protein has a cytotoxic effect onE. coli, and it is the cytotoxicity of thegp41 gene product that contributes to the high mortality when expressed inE. coli.