In order to find specific acting site of Rec A protein in plasmic polymerization in E. coli, we randomly deleted various part of pEC-3 (a derivative of pBR322) with SI nuclease treatment. Self-ligated plasmids were introduced into E. coli WA802(Rec $A^+$). A number of colonies were analyzed if they contained monomeric or polymeric plasmids by gel electrophoresis. The plasmid (pEC-43), which was deleted the region of tetracycline gene, revealed only monomeric form in Rec $A^+$ E. coli. When two plasmids, pEC-3 and pEC-43, were co-transformed in the same E. coli, the original pEC-3 showed polymerization but pEC-43 revealed monomeric form only. These results suggest that Rec A protein requires the specific site for polymerization.