PURPOSE: To produce the objective genetic product in the cells of host bacteria, in high efficiency, by using a vector in which the gene relating to the bacteriolysis of phage DNA is varied not to cause the bacteriolysis of the host bacteria.

CONSTITUTION: Host bacteria having lysogenized recombinant DNA wherein the DAN fraction having the genetic information concerning the production of coat protein of temperate phage DNA is substituted by the DNA fraction having the objective genetic information, and the gene relating to bacteriolysis, e.g. S-gene is varied not to cause the bacteriolysis of the host bacteria, are cultured under the conditions to inhibit the induction of the autoreplication of said recombinant DNA, e.g at 30W38°C for 2W24hr. Thereafter, the autoreplication of the recombinant DNA is induced by heat induction at e.g. 40W45°C, and the cultivation is continued at a temperature to effect the proliferation of the host bacteria. The objective genetic product accumulated in the bacterial cells is collected therefrom.