Abstract: A replicon, in which a nucleotide sequence encoding a cell killing function is regulatably expressed when the replicon is harbored in one type of host cell (primary host cell), so that cells harboring the replicon are killed under conditions under which the cell killing function is expressed, and the nucleotide sequence encoding the cell killing function is regulatably or constitutively expressed when the replicon is harbored in another type of host cell (secondary host cell), so that cells harboring the replicon are invariably killed or killed under conditions under which the cell killing function is expressed, may be used in a method of active biological containment of cells under defined environmental conditions. The biological containment principle may be utilized in the industrial production of a biosynthetic product by recombinant DNA techniques, when deliberately releasing a genetically engineered microorganism to the natural environment or in the preparation of a live vaccine.