E. coli LE392, transformed with CDDP-treated pBR322 DNA, was plated on ampicillin containing media. The number of colonies formed on ampicillin containing agar plate was reduced to undetectable level after trot the DNA with 13.3 μM CDDP. The CDDP- treated pBR322 DNA was susceptible to angle strand DNA specific Sl nuclease and it`s migration pattern in agarose gel electrophoresis was changed. These results suggest, that CDDP adduction to pBR322 DNA resulted in denaturation of the double helix and changes in it`s conformation which ultimately leads to the inactivation of the ampicillin resistant gene(Received November 25, 1990, Accepted December 22, 1990).