Various mutants of an Escherichia coli K-12 strain were prepared, in which a chloramphenicol (CM) resistance gene (cml) derived from an R factor, R100-1, was integrated into the chromosome. The CM acetyltransferase (CATase) activity of these strains and the strain carrying R100-1 were determined during exponential growth with the following results. (i) The CATase activity varied, depending upon the site of integration of the cml gene on the chromosome. Activity was found to be higher when the cml gene was integrated nearer the replication origin of the chromosome, the total enzyme activity found was the sum of activities coded by each gene separately. (iii) When the cml gene was in a cytoplasmic state on an R factor, R100-1, the expressed enzyme activity was four-to eightfold higher than that in the chromosomal state, suggesting the existence of about four to eight copies of R factor per chromosome. The CATase activity returned to the level of that expressed by R100-1 when the chromosomal cml gene was detached and picked up by an R factor.